Should I add a return or another supply to a large room with only one supply?
Adding a return and/or additional supplies may help. Adding a supply and a return would almost certainly help -- it might be overkill, but you could always close them off if needed.

The important point, however, is that this is a question of balance relative to other rooms. Knowing the best solution requires understanding the layout of your home and the sources of heating/cooling load. You mentioned this room has 3 windows; are they exposing this room to more solar heat gain than other rooms? Is it properly insulated? Are other rooms over-supplied? Is your home's entire cooling system undersized? All these factors should be considered before you make a change like adding ductwork. It could be that you need insulation, curtains, or even something as simple as closing vents in some other rooms.

There's a fair bit you might learn with some experimentation. Someone with HVAC expertise who can walk through your house would likely be able to evaluate quickly and suggest reasonable options. You could also have someone do detailed heat load calculations, but that may be overkill for your situation.