The gcode you need is G28. This takes the machine to s predefined position, which could be your tool change position. You set the position (using machine coordinates) in one of the Mach3 configuration screens, although I can't remember which one off the top of my head. As m_c says, you can then incorporate this code into your M6start macro so that as part of the tool change operation, the machine will go to the tool change position. On my machine, I also have a fixed touch plate set into the bed at this point so I can do a "second operation" tool height setting without having to jog the tool again. When you hit "run" the M6end macro is called which by default takes the machine back to where it was when the m6 tool change started.