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Both those MOBOs use Dual/Quad Channel configurations. While you 'can' get a Tri Channel kit that's either Certified or Tested you can ONLY use 2 of the 3 sticks, and No you cannot buy a single stick to make a Quad Channel the will work 'properly.' Theory vs Patrice; the kits aren't matched just for the heck of it.
Instead go to to MOBO site and D/L the Certified 'Memory List', use the RAM Mfg's online Configuration, or submit a Ticket to the RAM Mfg prior to purchase.
I would only recommend you buy 4X__GB kit, 2 exact -> 2X__GB kits or one kit. Also, the CPUs with 6-cores often have their 'own list' of RAM.