When you say "together" do you mean in a spagetti form one after another or in a star form?

The enter of the star is where the power supply (-) should be connected.

All other connections and configurations are creating ground loop currents.

Also if possible use this kind of connection fr the (+) of the other pcb's too. The star topology is very important to minimize interferance. Thena good decoupling locally is also very helpful.

Ioannis