Hello,
Unfortunately, the 3pi is not compatible with the 42x19 wheels. Are you trying to add encoders to your 3pi? Our encoders and 42x19 wheels are compatible with the rrc04a chassis. If you used the rrc04a with one of our Orangutan robot controllers, a lot of the hardware would be handled for you. You would be responsible for connecting the encoders, the motors, the batteries, and any other additional sensors you wanted.
You might check out another customer on the forum who is trying to make a micro-mouse-like 3pi using distance sensors instead of encoders.
- Ryan