This photo of the XB-70 Valkyrie's main gear shows a small wheel between the two main wheels:
Courtsey of AirPigz.com
This smaller wheel doesn't touch the ground, though I can see where it might as the side walls of the main tires compress under landing weight.
The AirPigz site promised an explanation, but none has been forthcoming (in the 4+ years since that was posted), and I couldn't find any information on it on Wikipedia, either. Does anyone have any information (real or speculative) on what it's for?