: Is starting In Media Res compatible with a Three-Act Structure? I have a story in mind which I am confident will fit a three-act structure very well. However, I am uncertain about the exposition

Can it be effective instead to move the Inciting incident to the very beginning?

No.
Obviously that is my opinion, and you may find some decent writing that has done that; good writers have broken just about every rule in writing. So instead of "rules" maybe we call them "guidelines" and common characteristics of what we usually see in best selling writing. The Three Act Structure (3AS) itself is derived from the analysis of extremely popular stories; it is not a prescriptive structure but a descriptive structure of the commonalities of what millions of readers regard as "good stories".
I think you have missed a crucial component of the 3AS. The first 10% to 15% of the story, before the Inciting Incident, is there to introduce the main character(s) and the world she/they live in.
Without that introduction, we don't care about the main character, the inciting incident doesn't really mean anything: We don't know what is normal, we don't know anything about the hero so we don't care about them, we have no emotional attachment to the story or characters in it and we don't understand the setting.
I am guessing, but I think your problem is that you are trying to slog through a lot of explanation of the character and it feels boring, your character is just "waiting around."
What you need, throughout your book but particularly in the beginning, is a little conflict; a little difficulty for her navigating her normal world and setting. This doesn't have to be a battle or life threatening, in fact it can be a very minor irritant, but still something she has to deal with.
So, for example, she wakes up without the alarm, which she usually doesn't, and looks to find her smartphone is on her bedside table but the alarm didn't work -- Because it is dead, and she realizes she woke up an hour late and is going to be late for work. Now she has a problem, she has to figure out why her phone is dead AND set a land speed record getting dressed for work and into work.
Or she goes to take a shower. It is winter, there is snow on her window sills, and there is no hot water, the water is arctic level frigid.
Or let's take one that can tie into the rest of the story: She goes to make cereal for breakfast. She pours milk on the cereal, and it comes out in glops; the milk is soured. Okay, dump that down the sink, no cereal for breakfast.
So, this problem causes her to break her normal routine, she stops at a pastry shop to get an Apple Fritter for breakfast. The guy in front of her in line turns for a moment, smiles, and says, "Hey!"

He sounded like he knew her, but Karen didn't recognize him. "Um, hey?"
The man grinned, then turned back to take his turn at the register. "I need two dozen glazed in boxes, and an Apple Fritter for the lady." He spoke over his shoulder. "Still an Apple Fritter, Karen?"
Oh my god, he knows me and I don't recognize him!

And this is not the "Inciting Incident", but the man turns out to be a guy that worked the snack counter in the Student Union coffee shop where she routinely got an Apple Fritter, and had forgotten. But now he is graduated, finished law school, and bumping into him is an important part of what leads her to the Inciting Incident. And this can also be a romantic interest, if we want, or this guy can be an important ally in the fight to come.
The first 10%-15% of the story is the MOST IMPORTANT part of the book, that is why agents and publishers want to see the FIRST chapter, the FIRST ten pages or so, because this is the most difficult part to get right:
You have to introduce your characters and the setting cold, and make that interesting, and keep the reader wondering what happens next so they keep turning the pages.
If you find that boring, you aren't doing it right! What keeps readers turning the pages (throughout the book, but especially to start) is conflict, meaning a character dealing with problems they'd rather not deal with. Malfunctions, mistakes, social awkwardness, the unexpected. Those have consequences, and those push the character into new directions as they try to solve these problems.
What the whole story is about, is a character dealing with a big problem, perhaps a life-changing problem, or a world-changing problem. The beginning of the story, BEFORE the Inciting Incident that will (in the next 10%, by the end of Act I) grow into this life-changing problem, is where you can show us the kind of world she lives in, and most importantly, how she deals with her common life conflicts. Those make us care about her and understand her. They establish both her unique skills, and her unique weaknesses.
Invent problems for her. Make the lights go out, and while she's searching for the flashlight, she knocks an open box of tacks on the floor. They scatter. Show us what she's like.
Stay away from the Inciting Incident; but you can subtly "mirror it" with her "little" problems in the intro. Karen meets Doug, now a prosecutor. She likes him, is even attracted to him. She ignored him in college, just another clerk behind a counter, but that was a mistake, Doug is a great guy, he's funny, has a fantastic memory, etc.
In the mirror: Her "Big Story Problem" will soon be the CEO of the place she works: She thought he was a great guy, but he has been doing the opposite of Doug: Rich and taking a free ride in college, and is now breaking the law, cheating on the books, on taxes, and involved with mobsters, all flying by the seat of his pants.
The Inciting Incident is the first thread she starts to unravel in this scheme, a discrepancy in the books perhaps.
I know that is not your story (and not a story I have written, I just made it up for this example). But that is the approach. Small "throwaway" problems in the beginning gain our attention and let us follow somebody in minor trouble, but those get resolved, just a page or two from the Inciting Incident, which usually happens about halfway through Act I. It is the most critical part of the story, if you can't write this intro and make it engaging, nobody reads the rest of the book. Including agents and publishers!
To begin In media res means to start in the middle of SOME action; it doesn't have to mean in the middle of the MAIN action. It basically means don't begin your story with pages of exposition about the world and describing the character or her appearance or background; that is boring. Start with characters doing something, on Page 1, Paragraph 1.

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You can definitely start a three-act story with an in media res opening. A story having three acts is more about the number of plot points and their location. Here's a good primer on the three-act structure.

However, should you start a story with an in media res opening?

You'd be surprised how fast you get from the first word to the inciting event (that should happen about 1/8th into the story).

The 50 or so pages up to the inciting event (in a 100k-word novel) are needed to establish characters, conflict, and setting.

Skipping this part and doing an in media res opening risks turning into "a car accident you read about in the news" as opposed to "a call about a close family member that has been in a car accident".

Before you know the characters them being in danger doesn't really concern you.

If you check out in media res openings in TV-shows etc (e.g. the classic main character in dire straits followed by an "X hours earlier") this usually only happens after we've gotten to know the characters, rarely in the first episodes or even the first season of a TV show.

Maybe you need to work on the tension?

Yes, you need to introduce characters, conflict, setting, etc... but if you don't do that to the hiss of a burning fuse it can become boring, unsatisfying or feeling like nothing is happening.

You don't need lots of conflict, but you do need the potential for conflict. Something that promises to become problematic soon...

Some examples; two people planning to win the same competition, incompatible personalities probably being forced to cooperate, especially to save their lives, I've talked about the Bear on the Beach technique before (placing a hidden threat to the characters in parallel with the main story, e.g. a robbery/kidnapping/murder being planned while the characters are living their lives as if they wouldn't soon be totally overturned), or a threat from the world the character is living in, e.g. a character living in a world where they have annual "games" where people kill each other and today is the day they select two contestants from each district...

Since the inciting event is (should be) the moment when the conflict becomes personal to the main character, nothing prevents you from starting to show the conflict before this moment, just more like a rumble in a sky with gathering dark clouds as opposed to a bolt hitting the main character's house and setting it on fire.

Another important aspect is to make all your introductions as actions (as opposed to telling). Imagine your character riding in his car and coming to an accident site. Does he stop to help, or does he drive by? What is he thinking about the accident? If he does stop, does he choose to save some people before other people? This will tell the reader about the character by his actions instead of using narration.

You should also make sure all your introductions are interesting... exploit your characters quirks or unusual sides to make the introduction original.

I think, once the conflict starts rolling, eccentric quirks and originality are much less important. They may even become obstacles to showing the conflict, but in the beginning, we need to meet interesting, unusual people presented in an interesting unusual way and at the same time feel the potential for conflict gathering in the air.

Update: Your intuition is right though. You do not have to do all presentation in the first act. Rather the opposite; do as little as you can get away with, while still having an emotional bond between reader and characters once the conflict starts getting personal.

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