You are confusing the memory type (DDR3) with the Memory controller interface (Dual/Triple Channel).
Prior to the release of the Intel Core i7, all ddr3 systems were dual channel. This has nothing to do with the memory itself, and everything to do with the memory controller used in the platform.
The core i7 has an onboard triple channel memory controller, which in combination with the X58 chipset allows triple channel memory to be used.
The core i5 and some Core i7's that were recently released use a dual channel memory controller. This is cheaper, and allows for cheaper motherboards that utilize the Intel's P55 chipset rather than the much more expensive X58.
Adding a third channel does not give a 50% performance increase. It does give a significant improvement, but not 50%. Pure memory bandwidth increases do not necessarily translate to 1:1 performance improvements. Some apps will see better performance than others.