Wade,
Mach 3 is really a machine controller. You will still need software that will generate your g-code for you. An inexpensive solution like meshcam might be good enough if you don't need 4th axis capability. If you need 4th axis have a look at deskproto which is still reasonably priced...cam programs can get very expensive. The overall 'software chain' would have you creating your object in moi and exporting the Stl files to meshcam that would generate the g-code that would run your machine being managed by Mach3. If all you want to do is make signs you might be able to get by with lazy cam that can be purchased as an addon to mach3-it can generate g-code from dxf files which are possible to get from moi (with that format converter).
hth...possum