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 "act" shows Gandhi as a young whippersnapper. We see Gandhi's time in India as a youth before he leaves for England. He goes to school, steals money for cigarettes, and does normal youthful things, including getting hitched (which isn't unreasonable for a 13-year-old in turn-of-the-century India).
The big change in his life comes when a family friend suggests he go study in England. Gandhi breaks with his caste to go there. If he hadn't bravely disobeyed his caste and left for the foreign country, he probably would never have become a lawyer, let alone a world leader. It all starts once he breaks away from home.
Act II
In general, an Act II lasts until the moment when the characters are as far from resolution as possible, and while this text isn't constructed like a fictional narrative—after all, fictional stories tend to be aimed at providing suspense and meaning, and this is a guy's real life we're reading about—you might consider the moment Gandhi is thrown off the train in South Africa due to color prejudice as the moment he's furthest from a resolution of his story.
When he's kicked off, he briefly entertains the idea of not fighting for his rights. The whole story could have ended right there, had he chosen right then to just go along with injustice like everyone else. But instead, he chooses to fight, taking us into Act III.
Act III
The rest of the book takes us through the author's various experiments with self-purification, such as the brahmacharya vow, which empower him in his political work. He also describes, to some extent, his different political actions, such as the famous Salt Satyagraha.
Mostly, as readers, we're left amazed by the sheer force of this guy. He ends by saying he still has yet more ground to cover, to become even more pure and a better devotee of ahimsa.