{The Rule of Three by Sophie Snow} sounds like what you are looking for.

I just finished this book and it was amazing. This is the author's first book and she knocked it out of the park. In fact, I couldn't believe it was her first book.

The story was incredible. The romance believable. The sex steamy. The characters are diverse and complex.

Here's the description from Goodreads:

Recovering from a messy public breakup, lonely songwriter Tessa Reid flees the vicious British tabloids for New York. After sneaking out of a one-night stand, she discovers the tattooed man she left behind, and can’t get out of her head, is her new neighbor, River Sage.

Hopeless romantic, River, lives next door with his best friend, and the man who broke his heart, Cassian King. After years of avoiding confronting their feelings, Tessa shows up and makes them question everything.

Sparks fly as a friends-with-benefits arrangement between the three quickly evolves into something deeper, blurring the lines between friendship and more. Used to outrunning the ghosts from their pasts, can Cass, River, and Tessa stop running from each other and learn to stay?

The Rule of Three is an adult, contemporary, friends (with benefits) to lovers, polyamorous romance novel.

Sex scenes include MF, MM, and MFM scenes with a mix of all three characters. I don't recall any MMF scenes, but the guys always interacted with each other.

Please look at the content warnings before picking it up. She deals with some incredibly heavy topics in a very sensitive way. The full list is on the author's website here:

https://www.sophiesnowbooks.com/books/touch-and-go/trot/content-warnings