You can practice with two balls, sound the same motions you would with three. And if you really want to focus you can practice making a consistent toss with one ball. Two is probably better bang for the buck.
Right, I was just pushing the idea that you can't always literally slow things down. That said, no reason you couldn't pantomime juggling really slowly. To be honest, I wouldn't be surprised if that is a legit path to getting going?