Solid advice Racer X as it comes to reading plugs, but I would be concerned about the three "black" ones in question here.
As the others are all ready a chocalate brown (not a bad color) under the same conditions as the three, I would have to ask: "Whats up with those three?"
GOT to use the same plug for every cylinder, thats for sure! Plug wires not delivering enough spark to the three perhaps? Assuming compression, valve settings etc are same for all cylinders.
Assume after a good run as noted above, ALL cylinders are now "brown". Will the three "foul" upon idling again? Valve guides leaking some
oil in those?