I installed iweb on my pismo running tiger a while back... (and a few G3 users report installing with no issues)
I remember inserting the install disk, the I copied the ilife package to the desktop. I ctrl-clicked it to view contents, located the individual application package under contents, ctrl-clicked to open, dragged the .dist file in contents folder to textedit, deleted the installation check script and saved it, closed everything and installed package from the desktop. I also edited the distribution file to tell it to ignore that it was a G3, just don't remember exactly how. 🙂
some of the other apps may choke G3s, and forget imovie, so be careful what you install this way, tiger and up only, etc. almost everything runs fast enough on mine, hope this helps someone... 😉