Flow should be exactly the same between all three GPUs as the distance between the inlet on the first card and the exit on the third card is exactly the same distance and flow resistance through each GPU block. Flow will be lower throughout all three blocks but due to the slighter lower overall restriction of the loop the CPU should see a slight benefit of flow. And I just moved from serial to parallel and GPU temps didn't give any noticeably change in temperatures.Just food for thought here.
Much personal experience with watercooling had taught me a lot about efficiency and cooling system theory. Might I suggest not having your GPU water blocks running in parallel.
Since your coolant will follow the path of least resistance. The flow coming from your entry port will likely send majority of your flowing coolant (forgive me if I am hard to follow) straight through to your 3 GPU and out the exit port. Basically I am implying that your third GPU is getting the best cooling out of the three cards. Yes there will be flow through the GPU 1 and 2 but it will be substantialy lower.
Not picking on your setup, it works great as it is. You probably would find a slight drop in temperature on your GPU's and "POSSIBLY" on your CPU and Bridge aswell due to a more consistant flow rate.
Food for thought.