I'd do parallel loop through 2 radiators and NB SB. Theoretically parallel reduce head pressure loss.
Pump out to GPU
GPU outlet split to rad 1 rad 2 then gathers into CPU inlet (this would be equivalent of large single radiator)
CPU outlet split to NB & SB, pump can have 2 inlets for return line from NB and SB (this would be equivalent of single NB SB block)
Might be worth if you're worried about single pump head pressure loss, might not be worth extra fittings & tube if it works serial just fine.