ok, my brain was on the right lines, but needed to to see it in practice
to do this manually you would need to mess with signature (basically use a common time base) or you can could get an approximation by using a fine grained zoom , and then using note offsets (33% for 2nd note, 66 for last)
however, there is an easier way, to do it on the pyramid!
create a temporary track in 3/4 enter the notes on that track, then copy n paste these on to your 4/4 track,
seems to work a treat… you are only using this for the lengths/offsets, you can put the notes in on the 4/4 track.
(btw: i believe the pyramid UI shows lengths as an ‘approximation’, so although it might say you have (e.g.) 3 notes of 6 length over 16, actually its using 3 of 5.333)