You can't combine separate PNRs into 1, but you should be able to call DL and have a phone agent assign your companion preferred seating based on your FO status.

You could also "link" the 3 PNRs together, but that does not really do much aside from leaving a note in the reservation.

Also see this relevant thread from earlier today: http://www.flyertalk.com/forum/delta...servation.html