I'm afraid that I don't know the answer to your scanner question but I suspect, as you say, that it's just a question of age. Scanners, in particular, seem to be badly supported once they are more than a few years old. (If it's any consolation this is just as true for Windows.) If it works on your G3 I'd leave it there - particularly if that's where you'll be running Photoshop. But, if you're feeling adventurous, have a look at
this link.
I'm glad that you have a no-cost solution to your problem even if it's not the most convenient one.