Can I hook up more than one device per terminal on a power supply?
The only reason there are three pairs of screw terminals is due to the high amperage output of the power supply. If you were to try to carry 50 amps over a single wire, it would need to be very large diameter and you would not be able to safely connect to a single screw terminal of the size used. If you check with an ohmmeter, with the power off, you will discover that all three V+ terminals are connected together internally (same with the V-).

If you wanted to have a single load taking all 50 amps, you could connect it with three pairs of wires, each large enough for at least 16.7 amps.

Similarly, if you had many loads to share the amperage, totally less than or equal to 50 amps, you could connect many wires to the terminals. If you have only a few pairs, you could connect them directly to the screw terminals. If you have more, you could run few large wires from the power supply to a distribution assembly to branch out to all of your loads. This could be terminal strips, buss bars or even a PCB. Take a look at the answer from @xakepp35 for ideas. (I believe it was downvoted because it did not actually answer your question of why.)

By the way, the reason there are three pairs is the same as why an ATX power supply has multiple wires of the same color (voltage) going to the main connector: too much amperage for single wires and too much amperage for single connector pins.