How would I check if a Player is in a part (Part.Touched, Region3 or other)

You have a few options to check if a player is in a part:

But if you want to make your life easier, you can use ZonePlus:

ZonePlus is very easy to setup and there are many features, such as:
-PlayerEntered
-PlayerExited
-LocalPlayerEntered
-LocalPlayerExited
-ItemEntered
-ItemExited
And much more.

I recomend using ZonePlus