Hello, Simon.
The 3pi was not intended for use with encoders. There aren’t enough free I/O lines to process two quadrature encoders, and there isn’t really physical space on the board to add them. That said, several people have found ways to add encoder feedback, but what they did was either pretty low-tech (e.g. using a reflectance sensor to look at the side of the 3pi wheels and trigger off the three spokes) or incredibly advanced (e.g. moving the motors and disconnecting the reflectance sensors to gain more I/O lines). There are probably some clever things you could do involving expansion boards, but generally that would be beyond what we could directly advise you on (though I’d be happy to provide you feedback if you want to run some ideas by me).
If you want a robot with encoder feedback, I suggest you make it from scratch using various components we sell. You can buy most of the 3pi components individually and put them together on one of our laser-cut chassis or something you design yourself.
- Ben