The difficulty with watercooling is that when you first start out, unless you buy a complete kit, all the terminology and stuff people simply expect you to know already can be a bit overwhelming. I have quite a few bits and pieces that I’ll never use due to them not fitting perfectly or other issues. You really should fire up the coffee maker and start reading a few warercooling FAQs - there are many of them out there.
The biggest problem is that most kits are geared towards the 1 CPU + 1 GPU combo, and with 3+ GPUs you’ll need a lot more than that. I live in Europe and I got in touch with Aquacomputer in Germany, asking enough questions to seriously test the limits of their very nice customer service reps’ patience, but in the end they helped me getting something that works.
In your case I’m worried the radiator is a bit too puny. My setup is geared towards a 3x580 + 1x590 setup, and I used their Aquaduct 360 eco+ external watercooling system and 12 V pump. This setup overheated so quickly I had to rush to shut it down. After discussions with client service I ended up adding another pump and this beast of a radiator. It has space for 9x12cm fans. So far I only put 3 fans on it, as it seems I’m finally keeping all the GPUs at a solid 60C (more or less) at sustained full load.
This might sound like overkill, but it really wasn’t. My first purchase was approx. the same as you suggest in your setup (3x12cm fans sized radiator), and it would’ve melted if I hadn’t stopped it in a hurry. So I think you need to scale up.
About pumps, the flow rate is less important than people think, but you need extra pumping power to overcome the flow resistance in your parts. Which means that if you were to buy the kit you suggest and then add another (external) radiator, it might make the pumps work too hard. I’ve also ordered a flowmeter. This is not because I need super fast flow, but because if for some reason the flow stops, I would like to stop the GPUs from creating a small bonfire in my PC case.
One final point: your fans and pumps are normally powered by your case PSU. I went and got myself an external power supply for the 360 eco, as when the system started heating up, it started sucking serious wattage out my PSU. Just something to be aware off. And remember that mechanical components like pump are more prone to produce wattage spikes (which will have nasty consequences if they peak above your PSU’s max), so be careful that you don’t move too far to your limit.
Hope this helps!