There are ten vehicles you can unlock in Twisted Metal for the PS3.
Here are unlockables vehicles, and how to unlock them.
Absorption Shield Beat Mr. Grimm Event #3 (Right + Triangle to perform ability)
Rear Freeze Beat Sweet Tooth Event #3 (Up + Triangle to perform ability)
Super Mine Beat Mr. Grimm Event #5 (Down + Triangle to perform ability)
Check out my other article for the full list of unlockables in Twisted Metal for the PS3.