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
From Ganelon's scheming to Roland's decision not to blow the oliphant. The Frankish rearguard is doomed! And so is France!
Act II
From the beginning of the Battle of Roncevaux to Roland's death on the hill. The flower of France has been cut down.
Act III
From Charlemagne's arrival to the end of the poem. Take revenge on Marsile, fight off the Emir, and execute Ganelon.