Someone said the other day that Richard Childress buys the #3 every year ,along with his other numbers, to prevent someone else from using it.At least until they decide who will use it in future.I doubt there are any other numbers that are held in reserve in that way.
The way I understand it the team licenses the number through Nascar evey year. If they don't use the number for a full year Nascar can take it and give it to another team if that other team request's that particular number. In the case of the #3 they allow RCR to license it every year without using it. I don't see anyone other than RCR running it ever again.
They might put a letter beside the number. I've had to run a letter with my name a few times. But just had it on the car once. I see it as a wastse of duct tape