Form 1
We see Baby like this when he is in Dr. Myuu's lab. He gradually gets taller/bigger as he continues to take energy from people. Around the time he lands on Earth is when he changes to form 2.

Form 2
In this form he is much taller and much stronger than his previous state. He is now strong enough to take on the remaining Saiyans. After he takes over Vegeta he takes control of every single person on Earth.

Form 3
We only see Baby in this form (his final form) for a short moment. This is when Goku shoots of Vegeta's tail making Vegeta a whole lot weaker thus Baby leaves the beaten up Vegeta tries to escape on a space ship. Goku shoots a nice Kamehameha at the ship blasting    him into the sun.