I am intrigued by your comment about crewing. I would assume you are referring to flight crew. On the transatlantic the only flights that have (had) 3 flight crew were the west coast. JFK/BOS/ORD etc are all 2 crew with a single overnight for the most part. There is the occasional 2/3 night longstop but that is down to rostering and to fit into the schedule and nothing to do with unions or pilot requests etc.
Actually, I was referring to both flight and cabin crew.
In any organisation, with obvious regard to regulations, work practices need to be realistic and not place unreasonable burdens on staff or the company. It was previously reported in 2005 that plans for a DUB-CPT route was abandoned because no agreement could be reached between EI and their unions over the rostering.
JAS