For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Bel Canto whips through Act 1 pretty quickly. Basically all the characters are irrevocably committed to the action as soon as the terrorists pop into the party and everyone realizes they're stuck there and possibly in mortal danger until further notice. In short, the book starts with the lights going out, then there are a few moments describing people's reactions before they know the terrorists are there, then the terrorists appear and pretty much catapult everything into Act II.
Act II
This book is pretty much all about Act II. There's a long, slow build-up to the moment when the military invades the mansion to shoot the terrorists and free the hostages. Things are as far from a resolution as possible right before the military gets in, and when they arrive there's a rapid blur of action that screeches to a halt with the realization that Carmen and Mr. Hosokawa have died along with all Carmen's terrorist companions. That's a hard Act II follow.
Act III
Short as the Epilogue is, it's basically Act III. Patchett wraps up the last details by telling us that Gen and Roxane get married and filling in what some of the other characters are doing. Plus, she cheers us up slightly after the emotional turmoil that was the end of Act II.