What? BOS-LHR is 2,829 nm great circle. Just under 3,000 nm for JFK-LHR. What transcon is longer than that? Longest transcon is around 2350nm (BOS-SFO and MIA-SEA are roughly that distance). I guess if you consider something like ATL-ANC (2,964nm) a transcon, you could say it’s similar, but you you said much longer (and I don’t think most consider that a transcon in the traditional sense, as it’s outside of the contiguous 48).

But I agree with your point that especially eastbound, 3 crew probably isn’t required. But there’s a good chance they would be needed for the return, and it probably isn’t worthwhile to deadhead someone (in mint) eastbound just to augment westbound, especially with so few flights and so little operational flexibility (in this regard with available crew block time), which could just be mitigated with augmentation. Either way, that mint seat will likely be used for either a deadheader or an augmented crewmember and won’t be producing revenue.