When a piston has three ring the bottom ring will be?

Most commonly the bottom ring will be the Oil Controlring.

(Control added by amclaussen.)

Oil control ring is correct. It is used to keep the cylinder lubricated as the piston goes up and down. The first two top rings are for compression. However, the 2nd from the top ring is also a wipper ring. Used to keep oil from getting into the combustion chamber.

(~Decker)