I have 3 keys also. I was told the purple is for the door, which is correct. I was told the silver was for storage hatches, and black was for the outside shower. But both the black and silver keys open all storage hatches and the shower.
I too have three keys. The black key is for the main outside door. The purple key is for the bedroom outside door and the silver key is for all other outside storage/shower doors. I guess it depends on how the key the locks.