If all of the units are powered they all need to run at the same pace. If they do not then you run the potential of the locos fighting each other and burning the motors out early. If the B unit is DC and the two As are DCC it is unlikely that you will be able to get them to run as a matched set. If it were me I would turn the B unit into a dummy or add a decoder to it. Why risk the As for the addition of the B.
TK, thanks for the information. I kind of figured that was my only option. So, for a while will run the back to back DCCs.