I'm flying JHDC with elite JHAS. Polaron beam build heavy into flow caps (Plasmonic Leech) for some great drains. PVE.
I just don't know which of the engineering tier 3 benefits the most. Marion sure helps for a few seconds against FAW/BO drain and stacking with the shield penetration is great. A2B helps recharge which is superb but the Plas Leech makes sure nearly all power levels are running Max most of the time.
I assume you mean Aux to Battery 2? There is no rank 3 in that boff skill.
The main point for A2Bat is to use the Technician boffs to reduce other skill cooldowns. You don't really need this in the JHDC because you have plenty of TAC boff slots.
I would suggest 2x purple DCEs for your EPT powers. Then slot A2Damp if you like and run the boff that extends the duration of resists with A2Damp -- it's a nice combo. M/AM I believe is the doff category.
How does A2B gimp the JHDC? Are you trying to make it a healboat? There are more things in the game than tac powers you know. You will have system cooldown on EptW, EptS, DEM/RSP, TT, ET, ST, BFAW/CRF and your Attack Pattern. You could even go half A2B and half A2D.