I can't speak directly about the Apple Firewire Kit, but the general idea behind putting a PCI firewire card in a Platnum G3 (take a good look it's an off-white that leans toward grey, they only turn off-white that leans toward brown if they sit in the sun or are very old) is so that you can hook up firewire devices, all of them. This category is most heavily populated by drives of many different flavors, with Dig8 and MiniDV camcorders and scanners coming in behind.
Knowing Apple, they have not developed the actual PCI card in the kit, but have purchased it from Adaptec or another interface card manufacturer. You don't necessarily need to go to Apple to get a quality firewire card for your older G3.
I hope this helps..