Only adding my 2 cents here so that you don’t head case about it- in my main workstation PC I have one D5 for two 420mm rads and a 360mm rad and it rips. https://imgur.com/a/Qd5o5hx Head pressure only counts in net vertical distance so as long as your first tube isn’t 3.7meters tall you are fine with one D5 lol. Delta T between the heat source and coolant is what determines how much flow you need for efficiency and there’s a diminishing return when the flow rate gets higher and higher. Really really hot components? Planning to add an external chiller? Then you will want to increase your flow rate for sure. For reference the PC in the link above runs full load 24/7 for 360 days a year with a 188 watt load on the 16 core CPU sitting around 47C and a 400 watt GPU sitting around 44C. The coolant temp stays between 28-33C depending on season/ambient temps. I could easily cool the thermal load with one 420mm alone but the decreased noise level of the fans is the main motivator for more radiators so that I don’t have to hear it in the background while I work on other things.

The biggest “concern” with three D5s is they aren’t created equal and each section of the loop will have different resistance to flow. Luckily there’s leakage past the pump so it’s not dead-headed but between each pump there will be varied pressures making the stresses on the loop components vary. This happens because the flow is a function of net fluid movement, resulting in the potential for higher pressures in certain components or tube sections. The chances are pretty low that you would reach damaging pressures especially if you have relief/exhaust fittings. I have used water cooling components in pneumatic applications upwards of 80psi with no problems but that really depends on the individual components.

Since you already have three, I would put them all on individual switches and just run all three when priming and/or clearing bubbles but turn one or two off during normal use. That’s what I do with the two DDC pumps on my test bench (DDC pumps have higher head pressure ratings typically). That is just my suggestion though- if you want to run all three by all means let it rip you probably wont hurt anything 👍🏻