You don't need to build three module wide. I maybe was not clear you need about 3 modules rated current if you want to use a series dropping reg, something to add if the + or- supply is well regulated all you will need is a dropping resistor(s) for the fil.
What is needed, the rated max loading of the rack supply. If you are in the total rated max current with the modules you have in the rack and the extra drain from the fil supply you might be OK, and as jbakker posted what is the B+ powered from?
The parts about the PCB and backplane traces is because I have seen backplane traces burn if a part drew too much current when failing on a card in other rack equipment so it is something I keep in mind.
I have not worked on or seen inside a API rack. Whats the rated current of the connector and PCB connector edge traces?