PS2 Tom Clancy's Rainbow Six 3 Cheats

Cheat: Stage Select:
On the main menu, press START to access the Campaing, Custom Mission, Split Screen, etc. choices. Enter L1, R2, L2, R1, D-PAD LEFT, D-PAD RIGHT, SQUARE, CIRCLE.

You should be able to select any map in custom mission mode (and play the practice missions on any difficulty).