Yeah, 3 is a very tiny amount. It's not as safe to ship only a few chicks...they need each other for warmth and safety. Check out My Pet Chicken, they might ship 3. The other option is to check your local feed store. My feed store (Steinhausers) will put several customer's orders together so you can get exactly what you want.
As for the terms you were asking about...
Pullets - all females
Cockerels - all males
Straight Run - males and females, however they hatched out (usually around 50/50, but could be different)