I'm trying to set up a Pentium with a PS/2 auxiliary-port mouse
  that has three buttons.  I seem to have Xfree86 configured properly
  (from /etc/XF86Config:)

You only think you have a ps/2 mouse.  The driver helpfully translates the
protocol for you, whether you want it to or not.  I'm using:

Protocol "BusMouse"
Device "/dev/pms2"

I once fixed the driver to deliver actual ps/2 protocol, but discovered that
the NetBSD version of XF has a broken ps/2 driver, so it didn't work (the
sense of the buttons was reversed).