In addition to the insufficient VRAM issue, another issue is your PCIE lanes layout of the motherboard (AsRock Z68 Extreme4 Gen3).
It officially supports 3-way CrossfireX and SLI (for two single GPU cards) and Quad SLI (for two dual GPU cards). This means, only 2 Nvidia cards are meant to be run together, at least officially. When running three cards, the mobo gives x8/x8/x4. AMD is a little forgiving for the x4 and thus this mobo passes the AMD certification for Tri-fire but not the 3-way SLI (for three single GPU cards).
Even if you can somehow enable 3-way SLI, the x4 will take away some valuable performance of the trio.
So, because of this and the VRAM issue, I do not recommend going 3-way GTX260.