Mach 3 is only ported (designed) to work with one processor. Therefore, it doesn't recognize any others regardless of how many there are.
The reason that I like using it, is that all of my other required system services files can and will be assigned by the BIOS to the other processors. In the beginning this was just an assumption on my part. It was after working with an Escalating Engineer at Microsoft on another problem, that the subject of CPU Utilization was broached. At this time I was informed that Vista was ported to multiple CPU systems to off load system service files to secondary processors. This is the reason that Multi-tasking is now possible whereas before with XP and previous versions it was implied but generaly not possible. At least not with the thought for speed of any kind. Personally I wouldn't run Vista on anything less than a Quad-core sysytem with a minimum of 4gb's. of memory.