i would get one pump but i would parellel the rads together and parellel the 3 water blocks together but that is me.
as a serial loop i would use two pumps one placed before the radiators and the other before the water blocks.
I run a single D5 with 3 blocks and 2 rads. It basically comes down to restriction and how your loop is setup. I would definitely parallel the GPU blocks if that's possible...will drop your restriction on the loop, overall.