I believe the versions of XNA and .Net listed on the webpage as requirements are the bare minimum required to run the game.
If your currently installed versions of XNA or .Net are newer versions, then P3D should still work fine. (I use the newer .Net Framework 4.6 and P3D still works)
As far as I'm aware, I don't believe it is necessary to uninstall and downgrade XNA or .Net to a previous version.
If you have the newest versions of XNA and .Net and P3D still doesn't work, It could be possible that your copy of XNA or .Net could be corrupted and need to be reinstalled, or you may be facing a problem from a completely different source.
If possible, describe or show screenshots of any error messages that may appear when trying to run P3D, it will help others in trying to help you find a solution.