Cheap or no cost software for tracking point's xyz cordinates in real time

Hi,

I am new to this 3d tracking. Was wondering if anybody knew of a cheap or free software product that would provide real time 3d tracking of a single point using multiple different cameras (most likely webcams)? It would probably be matte based (subtracting a color to be able to track it). For example, tracking a green ball around and having the computer able to output its coordinates in real time. Any info would be greatly appreciated.

Thanks, Tom