This from Wikipedia:
Triple-channel architecture
Operation
DDR3 triple-channel architecture is used in the
Intel Core i7-900 series (the Intel Core i7-800 series only support up to dual-channel). The LGA 1366 platform (e.g. Intel X58) supports DDR3 triple-channel, normally 1333 and 1600Mhz, but can run at higher clock speeds on certain motherboards. AMD Socket AM3 processors do not use the DDR3 triple-channel architecture but instead use dual-channel DDR3 memory. The same applies to the Intel Core i3,
Core i5 and Core i7-800 series, which are used on the
LGA 1156 platforms (e.g.,
Intel P55). According to Intel, a Core i7 with DDR3 operating at 1066 MHz will offer peak data transfer rates of 25.6 GB/s when operating in triple-channel
interleaved mode. This, Intel claims, leads to faster system performance as well as higher performance per watt.
[14]
When operating in triple-channel mode,
memory latency is reduced due to interleaving, meaning that each module is accessed sequentially for smaller bits of data rather than completely filling up one module before accessing the next one. Data is spread amongst the modules in an alternating pattern, potentially tripling available memory bandwidth for the same amount of data, as opposed to storing it all on one module.
The architecture can only be used when all three, or a multiple of three, memory modules are identical in capacity and speed, and are placed in three-channel slots. When two memory modules are installed, the architecture will operate in dual-channel architecture mode.
[15]
https://en.wikipedia.org/wiki/Multi-channel_memory_architecture
I seem to remember reading somewhere that not using three DIMM's would result in 10-15% memory performance loss. But it was a few years ago, and whatever.
