Recovering files from an iMac G3 hard drive (Mac OS 8.6)
From what I can tell the drive from an iMac G3 is an IDE type. HDDs running Mac OS System 8.6 ought to be running on the HFS+ disk format.

If you get a USB HDD enclosure with internal IDE connectors you should be able to plug the drive into it, connect the enclosure to a modern Mac and see the disk contents.

If you don't have access to a modern Macintosh you should be able to read HFS+ formatted drives under windows with the software hfsexplorer.