You can do your designs in autocad and then export to other programs to create code. Mach 3 is a machine controller. I believe they have a plugin to convert dxf files to g-code that is used to cut the parts. This is no good for 3d but is fine to cut 2d shapes. You should probably go that route and worry about 3d later when your more familiar with the flow of things. Code is not created automatically you need to tell the software what to do. First you select the geometry you want to cut in the order you want to cut them. Then you setup what the part needs to be cut. Height above part to move between cut, z value of top of part, z value to cut into part, speed to cut at and speed to plunge into part at. There is a lot to learn but it's pretty straight forward. You'll have to get the program and read the documentation. There are many program choices for you to choose from for creating cut files. Look up cam programs or cad cam and you'll be presented with many options. Good luck.