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
The first act of The Fountainhead basically follows our boys Keating and Roark, from the time they leave architecture school until Roark meets his icy beloved, Dominique. We see Keating the shmoozemonster get in good with Guy Francon, and Roark takes a modern-day apprenticeship at Cameron's broken-down architecture firm. Keating ascends the social ladder and Roark bottoms out, eventually working in a quarry and locking eyes (and lips, ooh la la) with Dominique.
Act II
This act is a stormy one. On the Roark side of the world, we watch as he goes through the Stoddard Temple fiasco and goes on trial. Don't you worry, though: he stays sharp and true to his principles. Dominique marries Peter Keating (who is turning into a big pile of complacent, talentless mush) even though she hates him. Once Gail Wyland shows up, though, Dominique leaves Keating in the dust and hops on the Gail train.
Act III
Things reach a fever pitch and then wrap up neatly in this act: Roark continues to be our unflappable, principled hero. He builds and then torches the Cortlandt House. He befriends and lectures Gail Wynard and continues to woo Dominique on the side.
Meanwhile, Peter Keating and Toohey conspire to bring Roark down, but Roark proves to be unsinkable. He is put on trial for being a firebug, but he uses his razor-sharp mental prowess to defend himself. This act (and the novel) ends as Roark is building a skyscraper for Gail Wynard and is happily married to Dominique.