The name of Transformers 3 is:
Transformers: The Dark of the Moon
Yeah. Too bad we don't really know anything about this movie, so the title would make more sense. Here's what Seibertron says:
Seibertron member Albershide has informed us that Amazon.com has listed 3 new books that scheduled to be released in 17th May 2011.
These new books are presumed to tie in with the 3rd installation of Transformers Live Action Movie. With this also revealed the title of Transformers 3.
The books are:
a) Transformers: The Dark of the Moon: Secret of the Autobots
b) Transformers: The Dark of the Moon: Autobots Betrayed!
c) Transformers: The Dark of the Moon: Invasion of the Decepticons
Also according to DomainTools.com, the Transformersthedarkofthemoonmovie.com has been created in 13th September 2010.