Mach 3 is strictly a controller, but comes with a few extras. LazyCAM, which can convert .dxf files from CAD and drawing packages to g-code, and it also includes a few "wizards", which will quickly create g-code for simple tasks.
Mach3 can be run without a g-code program, in MDI mode, where you manually enter g-code 1 line at a time. The machine will execute the commnads as you type them in.
I'd recommend downloading Mach3 and start learning how it works, and how to use it. If you don't have any drawing packages, you can get a Inkscape for ree from here,
http://www.inkscape.org/ and a free CAD program from
www.A9tech.com. If you plan on using Inkscape, I'd recommend also downloading the .dxf converter from A9tech as well and converting Inkscapes .dxf files to version 12 .dxf files. You'll have much more success when importing them into LazyCAM.
SheetCAM will give you more control when converting .dxf files to g-code.