All three are from bovines- cattle. A cow is a female, a bull is a male and steer is also male, but has been castrated. The main difference in the tonal quality of the skins comes from the level of the testosterone in the animal. Testosterone in bulls produces a thicker, denser, tougher hide.
When it comes to the rawhide, there is very little difference between cow and steer. Most tanners will tell you they can’t tell the difference between them after the hide has been processed. The way that they are processed can make more of a difference in the look, feel and sound than the sex of the animal. I have found that mostly what people want when they ask for cow is a skin that looks lighter in color. The color is due to processing with lime or certain other chemicals, and it also gives a different sound characteristic than the way our steer skins are processed.
Bull rawhide is normally much thicker and harder than steer or cow. I don’t normally use bull for drums because it doesn’t resonate as freely, is harder to get a good sound from, and can be very hard on the hands.