To keep the load at 8ohms you might be limited (don't know the Univox prmary Z so can't recommed mismatching loads).
Cathode bias usually means high current, so I'd go 2x10", if you have room to place one speaker directly above the other, more the better. Speakers will need to be 2x16ohm in parallel, or 2x4ohm in series.
3x16 ohm speakers in parallel will make 5.3ohms...possibly a close enough match to 8ohms, taking in to account original caveat. 1x10 + 2x8 might be fine.
I'd personally avoid 12", but that's just me, others have made them work.
Just one question, if you like the no name 15" so much , why change it?