Hmm.  DRBD does support 3-node architectures, but it's asymmetric based on stacking, so it would require extra work, and anyway seems to be intended for a multi-tier approach involving a WAN or backup server so probably doesn't make sense for us.  Strange - I had somehow got it into my head that DRBD allowed multiple slaves.  I guess I was just confusing it with Pacemaker's functionality.