Three channels worth it......... in a word.........No!
That would be overkill not to mention very difficult to adjust. Look at it this way:
You would have three separate channels of water all going to three different blocks with different resistances. Say your NB block has less resistance than your CPU block but more than your GPU block. In that case more water will flow through the GPU block than the other two. On top of that, less water will go to your CPU block than your NB block becuase it now has a lower resistance that your CPU block.
To fix that you would resitrict the GPU block but now you have just changed all the resistances to the whole system and have to start from scratch again!!!
It just like trying to tune a 6 cylinder engine that had 6 independent carbs. you change one and you have to fine tune the others to compensate (a good firend tried to do this once with a Jaguar, he never did get that thing to run right)
IF you had enough time and constantly worked with it, you COULD adjust it for optimal cooling on all units but the hassle would not reap very big rewards.
I would just do two channels; one for the CPU, one for GPU and NB in series w/ ball valve if your CPU block has a lot of resistance to it.