[8-Ball]

That should work fine, provided you make the manifold as unrestrictive as possible.

Are you making the HDD waterblocks yourself or buying them.

As for flow balance.

I would say that the HDDs probably need the least cooling, so ideally, you want less flow through the HDD blocks when compared to the NB and GPU blocks.

Of all of the secondary blocks, the gpu is probably the most important.

I would be tempted to run two of the HDD blocks in series, and the NB with the other HDD block also in series. They will get sufficient flow to keep these low poewr devices cool, while the gpu block will still get enough flow to keep it as it will most likely be the least restrictive path.

By running the three parallel paths, your overall system resistance to flow will be reduced when compared to running the whole lot in series, so you will be getting a decent amount of flow through the cpu block.

Hope that helps.

8-ball