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
Fowler is uninvolved. He sort of befriends Pyle and maybe sort of loves Phuong, but he hasn't given himself fully to either relationship. He's as distant from them as he is from the battles and bloodshed around him. He doesn't take sides. Unfortunately, Pyle comes with a pile of trouble, and with his entrance into Fowler's life, Fowler cannot remain unengaged. He resolves to get involved.
Act II
Fowler gets involved. As Pyle makes a move for the hand of Phuong in marriage, Fowler takes steps to keep her. He writes to his wife asking for a divorce and writes to his employer asking for permission to stay in Vietnam. Fowler gets engaged in the war as well. He lures Pyle into a trap that will take his life and bring an end to his bloody meddling, not to mention his rivalry. Nowhere to go now but up, right?
Act III
Not so fast. Fowler regrets getting involved. With Pyle dead and Phuong his to marry, you might think that Fowler would be on top of the world. Unfortunately (or perhaps fortunately), his conscience troubles him. We leave him with his troubled conscience.