There's a Bavarian Division 3... D can be Depot Bavarian Depot III, could also be Backerei or better, Brüden Depot III etc, it's anyway probably a divisional or Armee unit, not a regimental marking. There's also a Feldbackerei Kolonnen 78 in the 9th Army, I suppose the answer should be found in the organizational charts of these big units, among the support units... interesting challenge ! It could be also just simply Bayerische Division III. and the 78 could refer to some support unit, part of this Division. D can also be Direction, there's for instance a ED (Eisenbahn Direction. B means also Betriebs, you could have a Eisenbahn Betriebs Depot III, in the 3rd Armee, this would explain the Guard plate on one of these helmets. It would fit with the Bavarian helmet also, since probably these units were common to an Armee and therefore sometimes with mixed contingents. I would be surprised to learn that there were more than 1 BA in the BAG. Last info, there's a Reserve Pioneer company 78 that was part of the 77.R.D and then 94 I.D, I don't know to which Armee these divisions were attached.... If they've been aattched to the 3rd Armee, then wa have a good option there...