Er... that's the only time I haven't found a manual at Apple. All the beige era monitors and boxes and bits that have gone through here, have had one or more manuals found there. I've got to give them five stars compared to other companies, and I'm usually pretty grumpy about Apple.
In this case, yeah we're missing the 'G3 Desktop' tech specs and user guide at least, but they did have the one about its optional A/V card. And for actual repair, I'd expect the minitower manuals would apply.
I wonder if you even need permission to host manuals that are freely downloadable?
I guess you would. The nature of copyright would include Apple's right to stop distributing. But meanwhile if you're concerned, what you can do is just download & sort everything you can get now. If Apple ever does pull the stuff offline, then all or part of that archive can be quietly distributed among the faithful on request.
As far as an official mirror goes, probably Apple would be leery of that. Corporations by nature like control, so their quick answer would be 'No'. But if they were approached correctly by a registered non-profit they could be fine with it. Like DigiBarn for example, if DB ever gets around to becoming a non-profit.
You might look around. For both a suitable organization, and just to check if some outfit has already made a mirror.
Like this.
http://www.virtualapple.org/manuals/
If nothing else you'll probably turn up a whack of good info and links that would make a nice sticky here.