3 Barbed water block?

Iv seen some of these around, but i cant seem to figure out how the extra flow drop the two hoses doesnt effect the flowrate somehow, as there is 2x water going in, 1x trying to go out.

Can anyone explain?

(P.S. looked around stickies but this Q hasnt been answered)

They are almost always one in (middle) to two out (sides).

yeah, inlet is always in the middle. if you really research pressure drop in closed-loop systems you'll find that its often the case that larger outlets versus smaller inlets actually decreases pressure drop thus increasing flowrate across that device. if you look at many radiators and heatercores, their outlet is larger than their inlet to increase flowrate across it. that principal would take me pages upon pages to explain with fluid dynamics, but its there. In the case of the waterblocks, you'd have to ask Cathar for a more definitive answer but it was my understanding that because of the small length of the flowpath in a waterblock flowrate between a one or two outlet design was nearly identical and any differences were negligable. hope that makes sense