Here is the sole cheat code I came across for Amped 3 on the Xbox 360.  If you have any more codes, feel free to post them in the comments or email them in and we will add them to the list.

Cheat codes listed below are entered at the cheat menu, under options. Select cheats, then enter the code:

Unlock All Sleds

• RT, X, LT, Down, Right, LB, LT, RT, Y, X