The official “Transformers 3 Push-Pops Board Book” gives it as follows.
The Autobots Bumblebee, Ratchet, Ironhide and Sideswipe led by Optimus Prime, are back in action, taking on the evil Decepticons, who are determined to avenge their defeat in 2009’s Transformers Revenge of the Fallen.
In this new movie, the Autobots and Decepticons become involved in a perilous space race between the U.S. and Russia, and once again human Sam Witwicky has to come to the aid of his robot friends.
There’s new characters too, including a new villain in the form of Shockwave, a longtime “Transformers” character who rules Cybertron while the Autobots and Decepticons battle it out on Earth.”
Of course, this is assuming the information for the pop-up book is accurate.