Why not just Transformers 3?

...or...

Transformers:Reign of Starscream (they could just borrow the title)

Transformers:Eclipse of Energon (made it up)

Transformers:The Last Autobot (borrowed it from LastAutobot)

Transformers:Coming of Unicron (mad it up but it's pretty bad)

Transformers:Spark of Cybertron (made it up)