I imagine the 3 hour window would be OK for a general dinner at a restaurant, but for the Luau it might be cutting it close, especially if you arrived a bit early for a look-see at the Poly etc. Maybe for the Luau they have some "special" parking pass?? Somebody will probably know--.
Should add that you do not have to have a PS somewhere to park at the Poly (or any other Disney Resort). Many do, and the guard will have a list, but if you just wanted to stop in and shop, eat, etc, you can do so. You'll get the standard 3 hour pass.