SSE3 adds about 13 new instructions to its predecessor, SSE2. The most notable addition is the capability to work horizontally in a register, as opposed to the more or less strictly vertical operation of all previous SSE instructions. More specifically, instructions to add and subtract horizontally in a register have been added. These instructions simplify the implementation of a number of DSP and 3D operations.
SSE3 also includes a set of Instructions related to HyperThreading. AMD have left out two instructions from the SSE3 instruction set, the FXSAVEFXRSTOR instructions.