I considered the BA12-3 as well. At this point the MG12T-V or the pair of BA10-2's will cover everything for the present, but if I need more volume in the future, the BA12-3 is at the top of the list to go with one or two BA10-2's.
I think you're set and won't need the BA12-3, although I'm sure it's tempting (I'm trying to convince myself).
I just picked up a MG12T-V and was playing around with it and my 2 BA 10-2s today and have decided that any combination of those 3 cabs (1 cab, 2 cabs, 3 cabs) sound great.
I also strayed from Jeff's advice and put the MG tweeter at 1/4 rather than off when combined with 1 or 2 BA 10-2s) Seemed a little richer (at least in my basement). I must admit, I do not know the science behind it; Perhaps I should listen to the expert.
Anyway, my options should cover any gig I have. If I ever go back to rock n' roll I may need to grab another MG. But for my pop/jazz combo, I'm set.