As long as you've made sure that it has OS 9.2.2 for classic (for classic & double-booting) and find out which is the latest OS on the G3 iMac. If it doesn't have OSX at all, then you are in for a long night, my friend!
You have to get the original OSX 10.0 somehow, and then build up your updates to at least Jaguar. Then you can do a straight upgrade to Panther, and then add the other updates to 10.3.5.
It'll take you ages to do, but it's totally possible. I know, cos it took me 2 DAYS to get it right. I say two days, it was two weekdays, and a guy has to work, you know?
Don't forget the security and firmware updates along the way, unless the most recent one will do for all. Which I doubt.