Learning GCode is a bit like learning mental arithmatic. If you don't know anything about it, how do you know when the machine has gone wrong.
If you Google GCode there are one or two sites that will give you an idea of the rules of GCode. However - it isn't that difficult to get started. G0 moves you in a straight line from here to your new destination, G1 does the same at cutting speed, G2 and G3 move you in a circle at cutting speed. That is all there is to it and with those four instructions, you can write all the programs you need.
As Hood said, your problem is not with the code but with the settings, and the main problem with all programming is starting at a position that everybody knows. Start the machine at position 0,0 with all offsets cancelled and away you go. Practise writing a few moves your self, or a short program. Leave the cutting tool out and watch the machine trace out what you have written. Another thing I do with some of the complicated bits is to put a piece of soft material - pine or soft plastic - and let my machine chew that up, rather than damage it on steel.
If you study the rest of the GCode list on Mach 3 you will find that the rest of the instructions are mostly to do with offsets and different drilling patterns - but the basic four movements are all you need.
Have a go - you can't hurt the machine - and you can't hurt Mach 3. You can, by the way, write a program and run it in Mach 3 without the machine attached. This, with the toolpath display, is an excellent feature that lets you see what the machine would cut before you even attach it. I use this feature on my office computer to check programs before going throuh to the workshop.