I have a 3 month male german shepherd. I'm just curious what's a good age to breed him with another german shepherd

Why would you breed him? Is he a pure bred? Does his line have any health problems? Can you track back over his ancestors?

I agree, there are plenty of professional breeders already. At 3 months you still don't know much about how he is going to turn out healthwise or temperament. Please answer Wondergirls questions too. Here is a good article to read about breeding http://www.essortment.com/guide-breeding-dog-22971.html