There's a lot of time spent WORKING on the computer. Playing games is often done on your own time, but you probably need to do this to stay on top of things.
Playing games for 3 hours a night (if that's all you're doing) isn't helping you learn to be a game designer because it's such a small part of the job.
When I was in high school I definitely played games 2 hours a night on average. I don't think I even get in 2 hours a month these days.