I had a long haul flight from DFW-SYD that had plenty of empty seats to the point they offered an upgrade to guarantee you'd be the only person in the row. Best spent $100 ever related to airfare.
This is or was a feature on Air New Zealand. You could buy three adjacent seats and call it a SkyCouch where two people can lie down and cuddle. Not sure how much the seats actually transformed into a couch. They were also very clear that clothes must remain on.
One time I was on a 747 YYZ→LHR with a total of 14 passengers.
I had that SF to Heathrow once, though I recollect four seats? Only time I’ve ever had a lie-flat bed on an aircraft.
3-4-3 and 3-5-3 are relatively common on 747 and 777 IIRC.