You are trying to do too much at once .. which makes for excessive amounts of stress and heartache. The secret is to keep things simple.
Try this approach, which is quite conventional.
(a) figure A to B and B to A as separate legs of the PNR flight but separate to the actual PNR bit in the middle. Note, in particular, the fuel burns for both of these legs. Those fuel elements get taken off the total fuel in exactly the same way that reserves do. That is, they are just another allowance you need to make for the sums. They don't come into the specific PNR calculation, per se. If the aircraft performance varies with weight (as is the case, say, for ATPL type questions, this is just a minor additional complication which can be handled easily along the way.
(b) So, as you head out over B, you have figured all the fuel bits which DON'T influence the final PNR calculation for the B-C leg. So, now, with the B to A bit addressed as a separate calculation, you have only to figure out the PNR bit for the B to C leg, and you do that exactly as you would for the simpler stock standard A to B PNR calculation. This approach, where you break a complicated calculation into its component bits so that you end up with a simple final calculation is a very common and typical way to run many calculations. In essence, you convert the total problem into a couple of tacked on legs with a PNR calculation, in this case, addressing B out to the PNR and then back to overhead B.
If it gets too complicated, come back to the discussion and we can fill in a few more bits of the gaps in the middle. Better you nut the problem out with a bit of a hand rather than we just show you the result. That way, you end up learning how to run the question yourself.
