It is a metaphor about the three ages of life and freedom,
the link below gives access to a free pdf book that covers this work from the first to the last moment of its creation: commissioning, development of the idea, concepts, sketches, selection, engineering, painting phase and assembly.
https://drive.google.com/file/d/0Bzc5miKRJrYOR2Z4YzlLMUxtVWs/view?usp=share_link&resourcekey=0-ysp3TakMfemNwaLDKXttEw
(please notify us if the link does not work)