Grey_Beard
The pump most likely will handle it, but I would do two loops. This would mean another pump. If you want one loop, here is what I would suggest for your loop: RAD1 - Pump - CPU - RAD2 - GPU - GPU - GPU -GPU - RAD1. A common rule of thumb is that each device needs 120 mm, which would mean you need 600 mm of radiator to effectively cool your configuration. I would suggest that the rad after the CPU be 120, then you will need 480 mm for the GPUs. I have three GPUs going to a 480. It handles it, but I think my 780 Ti runs a bit cooler than the 670, but I am not sure. My temps are in the low 40s when benching or folding, may get to 50, but not much more.

Grey_Beard
I considered that configuration for you, but that would require more tubing and no bridge for the GPUs. If you are okay with that, then what you suggest should be fine. I would be concerned with two things, (1) making sure the tubing is not everywhere and (2) that you have good air flow for the three rads. I did push/pull on my two 480 mm rads, but my case is really big. Let me know how it turns out. I need to drain mine over the next few weeks. Make sure you have a good way to drain the loop designed into the build of the loop.