Hello
Perfectly done!, just you need to load 5V line of PSU B , because all of the 750W PSU I know are single conversion (regulates on the 5V and if the 5V is not loaded, the 12V line will swing wildly usually between 12,7 and 11, 3V which it's not good for the poor three GPUs
Explanation of the single conversion / dual conversion PSUs:
most psus (specially cheapo ones) has several secondary windings on the transformers to obtain 3.3V 5V and 12V, this is done to keep the costs down, but the bad thing is, that the PSU cannot deliver ALL of its power on the 12V line.
since it is expected that you will use it on a PC , the regulation (feedback to the HV LIVE section) it is done with the 5V line.
if you don't load the 5V line the regulation of the PSU will be crap and will swing all the time , sometimes killing what it's connected on it.
some PSUs tolerates better no load on 5V than others, but there is no easy way to know that, except testing them, and specially dangerous is during power on (ringing effect) that it's when most of the damage to the load ocurrs.
Dual conversion PSUs are usualy the HIGH END ones, the transformer has only ONE GIANT winding to obtain 12V ONLY, so ALL of the power can be converter to 12V directly, to obtain 5 and 3.3 they use separate VRMs (usually on a vertical board) like the famous superflower design.
since this power supply is essentially a 12V PSU, with extra gadgets to obtain the other voltages, it regulates on the 12V line itself and it is perfectly safe to only load 12V actually you can load whatever you want because if you only load 5V those are obtained from the 12V anyway by the VRMs so, it is the same thing!.
dual conversion are usually 850 W and UP, with some unusual exceptions like the coolermaster 750W which is dual conversion too.
it is easy to know just by looking at the low voltage side, if you can see a BIG vertical board with coils and VRMs then it is dual conversioin
But you already have your setup working, just find something that eats 1A on 5V and you are set
1A is usually enough to keep it stable.
cheers
Indtk.