Dc to DC converter: "split" one power supply (24V, 5A) into four power sources 5V, max 3A
Your LED drivers are controlled by the same micro, so everything must have a common ground.
I don't understand the &quot;series&quot; confusion, there is nothing to be wired in series here.
What you need is a 5V power supply for 4x 3A loads, so a 5V 12A load.
You can replace your 24V power supply with a 5V 15A mains power supply, or use a sufficient number of step-down DC-DCs to get 5V at the required current. For example if you get 5V 3A DC-DCs you need four, one per LED board.
Please do not buy the famous cheap counterfeit LM2596 modules from the internet, every component is fake, starting with the LM2596 obviously, but the caps are also garbage and they will die.