if you put PC66 RAM in PC100 slot, it might fit, and in theory, should load, and everything will run slow (and PC66 might burn out)...The RAM chip itself might have ratings on it, (ie I got a RAM module where the chips are actually PC133 rated, so I'm happy ^_^) otherwise, I recommend that you do some digging around on the info of those RAMs...
You say you know it'll work on a G3 but not sure if it'll work on yours? the Beige G3s use PC66 and Blue&White use PC100, just for your reference.