Depends...... I guess my question is do I keep my 3 phase loads separate when converting to kva from my single phase loads?
First, as kingpb pointed out the possibility of the POCO wanting to convert the system to wye...
Second, whether the high leg can deliver full service rating current. No one has yet mentioned the possibility that POCO may be supplying power via an open delta configuration with a lower rated transformer for the high leg. If this is the case, you will have to apportion three phase, two-pole, and split phase loads to determine loading on each line.
Finally, barring any additional post I may make to this thread, by all means, utilize demand factors in your calculations.