This isn't necessarily true in all cases. Assuming relatively normal parameters for such a system at idle with an acceptable flow rate I wouldn't except idle component temperatures to increase at all.
The thermal conductivity of the liquid will be fine and idle wattage isnt likely to come anywhere near to overcoming the ability for radiators to dissipate heat.
It's all inconsequential at idle anyway, especially if the fan profiles are dynamically configured against the fluid to ambient delta (as they should be really).
I wouldn't bother with a multi loop setup for what is a very vanilla custom loop configuration if temperature is the only consideration. If you are constantly changing GPU I can see the benefit of not needing to do a full drain and refill but otherwise it seems unecessarily complex.
Let assume his delta is 5C. Similar I managed using 3 x 360/45 rads. Adding the GPU, in idle, another 15-20W. Maybe 1C increase in delta. Under load, any Mid-high end GPU will dump another 200-300W in the loop. The question is how close his RAM is from throttling as it is now, as if coolant is at 40C, doesn’t mean the components being cooled will be close to it. GPUs, when good block and good contact is achieved tends to run closer to coolant temperature than CPUs and other components. If his actual coolant temperature is far from causing the RAM to throttle, adding the GPU should be fine. Just be aware that, even if the CPU won’t thermal throttle , it will idle at higher temperatures than now (despite the minimal increase from the GPU in the loop), and the temperature under load will be higher too, even if the loop is not struggling to cool everything. Dual loop can be a pain, extra cost for extra pump/reservoir, but isolating the CPU from the GPU will give you better temperature.
Just keep in mind that the best temperature is not the “needed” temperature.
My obsession with lower temperatures forced me to abandon water cooling as I spent more time/money changing parts than using the PC. It was my Lego.
The other consideration is loop order, as it won’t affect much, but I always used mine as pump/reservoir, bottom intake, GPU, top exhaust, CPU and back to pump/reservoir. Exhausting after the GPU helped the CPU slightly. Not a dealbreaker but as mentioned, obsession here. I was using thermal sensor before and after every radiator. In general won’t change a lot but was measurable.
Third Rad only needed if you want to lower your delta, when possible OFC. If delta is low already, and fans aren’t al high rpm, no point doing so.
