Welcome aboard Jamie, we all started the same way. Try to look at the 3 programs as doing different tasks and are only connected by how they are used in operational steps. the first step is the drawing of a part, Inkscape and Fusion 360 does that step. File saved in DXF or SVG format, opened in SheetCam and the operation is planned (selection of tool and how it is to be used). File saved as a G-code (.tap and other names)then opened in Mach3 for the actual table operation. firing the plasma and moving around the metal to cut. Inkscape may be the simplest to start with draw a simple box or circle, save it and move through the other steps to finish a cut operation. This will help with the proof of concept and boost your confidence (happy dance time). Select either Inkscape or Fusion 360 as your drawing program and stick with it until you are comfortable with how to create a file to cut.
Both are very powerful programs and go about the task in a different way to produce a drawing. You will find you will use both as you do more things for plasma cutting. Both are easy but you will need to get the big picture to understand how they work, to make learning easy. Lots of tutorials posted here and on YouTube to help you along the way.
have fun
