To answer the 2nd last question first, the Ubuntu iso disk image is
usually designed to fit onto a cd so you should have no problems there
(assuming the thing works). If you have problems downloading or
burning the iso (disk image) to CD, then you should be able to request
a cd from Canonical (through the Ubuntu website), which they usually
send out free of charge.

As to whether they still officially support the old G3, or just the
new Intel Mac's, I couldn't comment - one for further investigation.