Elysium Alps- There's 1. When you past your 1st checkpoint, you'll see a
DANGER sign to the right. Go inside and you'll see the Speed Boost.

Mercury City Meltdown- There's 2. When you take the shortcut that's in
the park, (marked with a SSX sign) don't jump when you need to. Just
ride and turn to the left. The last one is... it's in another cheat.
Happy?