I suggest that it's more a case of .. when things are going well or not too badly .. it doesn't really matter whether it's a well-oiled 2- or 3-crew operation.
However, if everything goes wrong .. three competent, well-trained brains, pairs of eyes, etc., etc. ... probably give you a better fighting chance than two.
More importantly, with the two-crew animal and, say, a new chum F/O starting his line training program and after the safety F/O (or whatever support is provided for the initial training program) is released ... the LHS is in a very awkward situation if a serious problem arises and the RHS is just not up to the requirement at his/her level of experience/training.
I wonder what Al Haynes would say about 2-crew ops in awkward circumstances ?