The cup car is a stand a lone series, you only race against other cup cars. The RSR should be the fastest, followed by the GT3 as they allow a-lot more aero and driver aids. The Cup car is meant to be a purer experience, shifting, braking, steering, all done without any aids and thus a car that is not competitive vs the other two.
Generally speaking, whenever you see a car listed as a CUP car, it belongs to a single make series, like the BMW M2 Cup cars. Lambo Super Trofeo 'COULD" technically also be called a cup car.