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
This play doesn't lend itself particularly well to a three act analysis. For starters, the point of no return has already happened before the play begins (that's the point; hence the title Endgame). You might call whatever the disaster was that ended the world and trapped them in Hamm's house the point of no return, but the disaster itself is not part of the play.
Act II
Here, we again appeal to the point of the play. Resolution is imminent – the play ends as soon as Hamm and Clov give in to despair and stop talking. Yet, the characters often act as if resolution is far out of sight. One point where this is especially apparent is when Nagg and Nell are talking in their trash bins, and Nagg repeats his joke about the Englishman and the trousers. They appeal to the past instead of facing the present.
Act III
The end of the play comes as Clov gradually begins to resolve that he will, in fact, leave Hamm (though we don't actually know if he leaves him or not). We would suggest that one key point is when Hamm narrates the story, which seems to be the story of how he obtained Clov as his servant (i.e., taking Clov from his father). Shortly after, Clov becomes very quiet and then gains some power over Hamm when he tells him that there are no more painkillers. It's impossible to know exactly when Clov's real conviction to leave begins, but this is one possible place.