Fortunately the GSD is only three months old, so he is at a good age to learn. I doubt that he displays the type of aggressiveness at this point which would kill a cat. BUT, it could develop over time.
Our adult GSD killed a cat in our front yard when I was growing up. It traumatized me, as I was helpless to stop it. Our family felt terrible. But those were the pre-leash law days when dogs and cats roamed outside freely and what happened, happened. I have never owned a cat in part because we always had GSDs.
NOW, that's not to say that all of my GSDs were aggressive toward cats, because they weren't. Much depended on the cat and how it behaved. When DH first took our GSD to his parents house, the dog returned home with blood drops on his nose. Turned out their cat was under a chair and the dog tried to sniff it. Essentially, this happened every time the dog went over there. He didn't hurt the cat, he was just curious. Sometimes he just sat there with the cat. And the cat didn't care because he was used to dogs.
Another time I was at my brother's house and the dogs were out in the fenced yard when DH called me to come take a look. There were my two GSDs laying on the front porch with a big orange cat sitting between them! It wasn't my brother's cat, either (he didn't have one then), it was a neighborhood cat who came into the yard despite the dogs. It was really funny! The dogs seemed to enjoy being there with the cat! (I wish so bad I had a picture of that!)
I think both of these situations could've turned out differently if the cat had run, because then prey drive in the dogs kicks in. So I think a lot is dependent on the cat.
When my babysitter's cat had kittens, we put all of them on the kitchen floor with the two GSDs outside the slider screen door. Most of the kittens avoided the dogs, but there was one who went over to them that wasn't fazed at all. We chose that one. (And ultimately couldn't keep it but the kitten and the dogs got along famously.)
I have a terrier now and if I could find a cat that wouldn't run from the dog, I'd keep it! All of us would love a cat!
So yes, I think, OP, everything you do with the dog has to be sort of "fun", but firm and consistent. If he starts to go after the cats, stop him with the leash and say No, and redirect him to what you do want him to do. Keep him looking "to you" for direction. As he learns this, eventually he will always look to you to see what you want him to do. This is what they talk about when they say GSDs need a firm leader. It will take some time, but he eventually will get the idea, and hopefully the cats will become more used to him, too. Just work it through. Don't let him have access to the cats unless he is on a leash. He could get hurt if he does get to one of the cats, and of course, the cat could get hurt, as well. Good luck! PS Pick up a copy of Good Owners, Great Dogs for lots more information on working with your dog. Would also love to see a pic!