This behavior has been going on for several months now. It is plain ridiculous.
Shop for a cruise using 3 guest occupancy and you will see all those "missing" cabins. Either book with a CVP (Princess) or a TA (independent) and they can get it done. Otherwise, book with 3 people yourself, put cabin on hold, call in to drop the third person. Or, just pay the 3rd person deposit with cash and then cancel them later and get a refund. Just stooopid, IMO, PCL!!!