No.. they don't have enough kick to run it... nothing like enough. n00buntu is way to bloated and heavy for anything but new hardware.
They won't even run the light linux distros, because generally those don't support the ppc cpu architecture. The debian ppc kernel in lowram mode will just about run, but it is very very slow.
I was given 20 of these 2 years ago.. after weeks of messing about they all went for recycling as they aren't worth the time and effort for the end results. My original comment is valid.. Give them away, install some old mac-os or scrap them.
Those old G3's were the CNC (controller) for 'strippit' brand (american made) turret punch presses. Go through your phone book and do a search on service technicians or maintenance company's that specialise in the metal industry's. Understandably G3's are thin on the ground and you just might get lucky....but you won't make a fortune at all.
That's interesting.. Mine came from an obsolete mechanical telephone exchange where they had been used as some kind of "dumb terminal" switchgear monitors/controllers.
I kid you not, these "suedo" controllers sat in oversized fibreglass cabinets, connected to an average garden variety monitor. Everything looked spiffy on the outer, but take off the top cover and you died laughing. There's one good aspect with regards to the macs, in such they were exposed to extreme conditions industrially but they seldom skipped a beat.
As for the presses themselves...they make great anchors apparently.
Found in telephone exchanges you say? We must have your hand-me-downs. Our telco's are fabulous, totally reliable. :P