How to wire together 3 power supplies for 3 different pd rails with the same reference 0v
If they are isolated PSUs, then you can set each one to the required voltages and connect them together. The two positive ones have their "negative" outputs connected together to be the 0 V rail, and the supply for the negative voltage has its positive output connected to the 0 V rail:



If there will be high currents between, say, -6 V and +12 V then there may be other considerations: please add a comment if that is the case.

Also, and a bit out of the scope of what you're doing, power supplies have a maximum rating which limits how much voltage you can create by putting them in series - you could not put a thousand 30 V PSUs in series to create a 30 kV supply. [The name of that rating escapes me - perhaps someone would be so kind as to add a comment with its name.]