There are two ways for you:
If you have one more Flight Simulator (FSX / P3D) on your PC and any flight planner for ones (Flight Sim Commander or other) you create the flight plan for „another“ simulator and let PF3 load this, when you are in X-Plane. I prefer to put the filght plan „by hand“ only, therefore I have always a paper copy of the plan and put it in the flight computer (I fly airliners only).
Another way: you create your plan on any flight plan pages in the Internet and save it in a folder, where PF3 can find it. Be aware PF3 must be able to read the format of the plan.