When I sat down for the first time and thought about how you teach a 3 year old to roll up a mat, I was a bit flummoxed. It seems like explaining all the pinching and pulling and making sure that you do the same thing with both hands...all that was so much to get through to a little brain. But I'll be darned if she didn't pick it up and pretty quickly too. Those first few folds are a bit tough, but the process is pretty forgiving as a whole.
She usually drops it at least once on the way to putting it up. I think just so she can roll it up again.