Your description is a bit difficult to understand, but lets see.
You have three items which require 15vdc power and one power source.
You are asking whether:
1 To loom all the +ve wires together, and loom all the -ve wires together in separate looms. So making two looms.
or
2 To loom the +ve and -ve wires together for each piece of equipment. So making three looms.
My answer is 2. I have seen horrendous problems where method 1 is used.
Switch one power supply of a dual redundant pair, and the other one trips.
Switch one Processor Rack (CPU) and is dual redundant partner crashes because the inrush current in the wires for one CPU induces a voltage spike in the wiring for the other rack. (5 volt supplies dipping to 4.5 volts for a few milliseconds.)
An awful system, a real bag of something brown and nasty.
JimB