BtB (Bad-the-Bunny) is an inveterate collector (he used to collect veterates but they made him stop). He is now insanely compulsive about accumulating a type of item common to all three of the things below. What's the item and what's he doing with them?

1. Small thingie atop soda cans.
2. Small plastic or metal or paper thingie atop (some) paper or fibre file folders.
3. ONE CERTAIN key from all his friends' computer keyboards.