A Portrait of the Artist as a Young Man is the debut novel by the great Irish modernist James Joyce, published in 1916. The book is both a coming of age story (or Bildungsroman) and about the artistic development (or Kunstlerroman) of the protagonist, Stephen Dedalus, who is something of a stand in character for Joyce. While it does have a defined structure, I'm not sure it can be easily divided into three movements. The book is more clearly divided into three chapters by Joyce.

What I think is a useful way of looking at the structure of the novel is the movement and growth of Stephen Dedalus, who is named for the figure from Greek mythology who constructed the labyrinth. For Stephen, he is striving to move away from his family, Catholicism, and the stifling culture of Ireland. There is both literal and figurative movement in the book, as Stephen and his family move to Dublin after his father goes into debt. Stephen then goes on a religious retreat and, finally, ends up at University College in Dublin. So, I suppose you can see the three movements as relating to family, the Irish church, and his own intellectual and artistic development. There's also an autobiographical aspect to this, mirroring Joyce's own evolution and development as a writer, whose work would put him at odds with the more conservative aspects of Irish culture. His movement as an artist would be away from conventional modes of writing and towards experimentation and new forms of literature.

You might also want to consider looking at Stephen Hero, which is the book that evolved into Portrait.