Is this the base 3Pi computer, or one with the expansion board (basically has a 2nd level on it with a replacement CPU) ???
But the answer is YES, you CAN do stuff with it, it's just a matter of how much you want to spend to get the hardware to interface them.
Some things you could do -
Assuming you could interface wirelessly, have the 3Pi report back it's movement and has the RPi use that to build up a map of the environment (or even the line if the 3Pi is doing line following.) Basically SLAM.
Use the RPi to add in more advanced features such as hooking up a webcam (assuming you can get the RPi on top of the 3Pi, powered, and interfaced ) and do vision and object detection + recognition to augment the sensors on the 3Pi...
Basically like I said, just depends on what you want to spend and what you want to do. But the RPi and 3Pi certainly CAN work together.