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.
Because Roots functions as a genealogy on a structural level, it's a little hard to squeeze into a basic three act structure. Hey, no one ever accused families of being neat and orderly.