> Our machine
> is a 3/60 and has the onboard mono framebuffer but no keyboard/monitor
> attached (we're using a serial console on ttya).   How do we tell NetBSD
> to use that instead of the framebuffer/keyboard?

You need to set up the serial port as a console.  This is most easily done
by changing the setting in the eeprom.  I don't have my eeprom map handy,
but you can look in /sys/arch/sun3/include/eeprom.h.  The comments in that
file are sufficient.

The 'q' command in the monitor ROM changes a word of eeprom, like this:

>q 1f
1f:00 ? (type new value here)
20:45 ? (type ^C to exit)

Since you are already talking to the box, doing the above should be no problem.
If you cannot get to the monitor, then power down and put the NORM/DIAG
switch on the CPU board in the DIAG position, then power up with terminal
connected to ttya.  The system will run self-tests, then offer to drop you
to the monitor.  Make the changes, then power down and put the switch back
in DIAG position.