The solution for a three-axis robot is trivial -- assuming you have position sensing and control, just drive each axis until it is at the desired value.
The problem gets a little worse when there are obstructions in the work volume; this will require some form of computational geometry.
It's impossible to understand what, exactly, you would need help with, though. Is this for school? At what level -- grade school, high school, undergraduate, graduate? What is the context and audience -- software developers, mechanical engineers, roboticists, classically trained artists, or what?
