Hi,
if this is your first CNC experience then I would recommend Mach4 rather than Mach3. Mach4 is still be actively developed whereas all development of Mach3 ceased seven years ago.
It rather sounds like you have a Mach3 ready USB connected external motion controller? The vast majority, especially the Chinese made USB controllers, work (mostly!) with Mach3 only.
You can run Mach4 as a parallel port. The parallel port plugin is called Darwin and a $25.00 license fee applies. It would allow you to use a Windows 7 or earlier 32 bit PC but it will lack
many of the reatltime supports that you may want in the future. So a parallel port works but not recommended.
There are seven manufacturers of Mach4 ready external motion controllers:
1) Ethernet SmoothStepper ($190) by Warp 9
2) 57CNc by PoKeys ( $150)
3) UC100, UC300, UC400 by CNCDrive ( $120 through $160)
4) Hicon ( starts at $600) by Vital Systems
5) CSMIO (600 Euro, has buggy Mach4 plugin, good quality but marginal support)
6) PMDX412, PMDX426 ($120 through $260)
7) XHC, don't know the price, severely limited plugin, avoid like the plague.
All have their strengths and weaknesses. The two standout, in terms of realtime supports, are the Ethernet SmoothStepper and the Hicon. Having said that the PoKeys, UCnnn
and PMDXnnn will all work well for three axis CNC machines, but may miss on THC, or backlash comp, or single point lathe threading for instance.
Some of these devices require a breakout board as well, the ESS and UCnnn series for example. A breakout board does not really add any functionality so much as it buffers the motion board
output signals and provides good (screw) terminations for wires to your drives and limits switches etc. They can be a simple as a C10, a one port (17IO's) bidirectional board for $23.00
up to an MB3, a three port (51 IOs) for $190.
So you can see there is a reasonable range of hardware on offer all designed to extract the best from Mach4.
I have been using Mach4 for seven years with an ESS (Ethernet SmoothStepper). I used to use C10 (equivalents) but now use my own design and built breakout board. My new machine
is all servo driven and I wanted some servo specific features, ergo I made my own.
Post a few pictures of you machine, we may be able to identify various bits that may inform your decisions.
Craig