With 3-way SLI some games will run into CPU bottleneck pretty fast unless you really make it heavy on the GPUs, that means resolutions of 1920x1200 and above.
You're not seriously suggesting 3X 580 with a 3.1 Ghz quad, right? You'll prolly need to run the 2600K @4.0+ to avoid CPU bottlenecking.
And if you do get a 2560X1440 monitor, you do realize the cost of that right?
With more GPU intensive games like Metro 2033, Battlefield BC2, Crysis and even Far Cry 2 with say 8xAA 3-way SLI does show noticeable gains. But as I said, you do need extremely high resolutions for it to work effectively.
Also, gaming with three monitors where you literally increase the GPU load by threefold, these are situations where a triple GPU setup makes sense.
What doesn't make sense obviously is the massive amount of money you need to spend on a setup like this.
The three cards alone will set you back roughly 1400/1500-plus the cost of the rest of the set-up, including three 2560X1440 monitors
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And then obviously there's your power supply. The very bare minimum you'll need to get is a good 1200 Watt PSU, and make damn sure that the PSU you choose has these 6-pin AND three 8-pin PCIE PEG connectors to feed the graphics cards.
When it comes to CPU bottlenecking, you could say overclock to 5 GHz+ as you asked, but overclocking requires additional voltage on the processor, if you need to step towards 1.5v or even higher your overall peak power consumption will rise significantly. An additional 100 to 200 Watts may not be out of the question at all. And overclocking it to such dramatically high levels would endanger system stability by pushing the 1200 Watt PSU close to its limits.
So basically, this is really silly stuff to do. But people out there do purchase setups like this I suppose. But with 3-way you'll run into heaps of problems like power consumption, noise levels and then the CPU bottlenecks and after all that, scaling may in any case may leave you disappointed.