First of all, forget about LinuxPPC.
LinuxPPC only works on PCI macs since it
uses the Open Firmware, a writable part of
the ROM that only PCI macs have.
MkLinux though, will work on the following
configurations:

1) A 6100/7100/8100 with a Newertech MAXpowr G3
2) A 6100/7100/8100 with a SonnetTech Crescendo G3
3) There may be others, but I don't know about those.

Discussion.

1) If you own a MAXpowr G3 card, you can use MkLinux DR3

   by simply making sure that the MAXpowr G3 extension loads
   before the MkLinux booter does. The only disadvantage is that
   the backside cache does not work in MkLinux, so you'll get the
   speed of a fast 603e processor.
   To make sure that the MAXpowr Extension loads first, do this:
   - Change its filetype (using ResEdit or FileTyper) to "scri"
   - Add a load of spaces to the beginning of its title

   the Crescendo G3 extension to use MkLinux. You also need MkLinux DR3,
   or later. Do the same with the extension as described above to make it
   load before the MkLinux booter does. MkLinux should then finally work
   at the speed you paid for.
   Note: At the time of writing, I haven't installed MkLinux on my 6100
        + SonnetTech G3 card yet. If I discover any trouble, I'll update
        this page as soon as possible. 
 

   but the easiest way to find out is simply to try it.

----

PCI PowerMacs and G3 upgrades.

As far as I know, those with G3 daughterboards shouldn't
have any problem using either LinuxPPC or MkLinux.
Since these cards don't need an extension to activate
the daughterboard, everything should work by itself.

As for PCI cards with G3 processors on them, LinuxPPC nor
MkLinux should give you any trouble. If your G3 card uses
an extension to activate itself, you will probably only
be able to use MkLinux, since LinuxPPC skips MacOS thus
skipping the initiation of the G3 card.
To make those cards work with MkLinux, make sure you make
the extension that activates it load before the MkLinux booter
does, by doing the following:

   - Change its filetype (using ResEdit or FileTyper) to "scri"
   - Add a load of spaces to the beginning of its title