Mach 3 already has the routine for the camera installed in "Plug In Control"
The basis of this is that a suitable camera can be mounted on the lathe, chuck, milling head etc i.e. any suitable place on your machine and focus on another place - i.e. mounted in the milling chuck and looking down on the workpiece. The software already has a set of crosshairs electronically engraved on the camera output.
The downside is that it doesn't always work, - especially with the modern USB camera working under Windows drivers.
The cameras usually need USB 2.
They seem to be alright if the camera has a disc with a set of drivers in that you can manipulate, but some, running on the internal Windows drivers - all Mach 3 says is unable to open the camera driver.