Billboard One: This movie is frustrating
Billboard Two: Because its story is badly flawed
Billboard Three: But the performances are great
Three Billboards Outside Ebbing, Missouri goes rogue after the first act/first third of the movie.
The screenplay evidently attracted some spectacular actors led by Frances McDormand and Sam Rockwell, and my guess is that the script by Martin McDonagh -- who is a terrific playwright -- was much different than what eventually made it to the screen. That's the only way I can imagine that such great actors would sign on to this Indie film. In my fantasy, the film as originally intended didn't test well and sat on a shelf until it was radically re-cut and then released. [Note: I have no idea whether this happened, it is my fantasy.]
The plot, such as it is, revolves around McDormand seeking justice for her daughter who was raped and murdered.