Act I takes us through the peaceful Summer session at Devon and ends right about the time Finny declares his best-friendship for Gene and Gene decides to kindly NOT return the favor. At this point, we're pretty much tied to the whole jealousy-fear-resentment plot line.
Act II then follows said jealousy-fear-resentment plotline all the way through Gene's machinations and discovery of Finny as better-than-he. It ends when things are at their worst – when Finny falls from the tree.
The rest of the novel makes up Act III: all the suspicions, guilt, and identity-switcheroos – you know the drill. It ends with Gene reflecting, fifteen years later, on the lessons from his friendship with Finny and those years at Devon, and how they infused his experiences in WWII and beyond.