You make your tool path in Rhino/Rhinocam and export as a G-code file. This is just a simple text file. You open the G-code file in Mach3 and this reads the instructions and outputs these to a CAM controller and that changes the information into pulses the stepper motors understand.
So you can bring the .txt file from another computer, download some G-code from the internet, or if you have the patience, write your own.