First clear ur confusion ! see, it is the function of graphics driver, not Windows 7 itself. Intel chip sets with integrated video controllers (hope u have the same, though u did not mention here the configuration) only support a maximum of 2 monitors. U will either need multiple discrete graphics cards, or will need to find a specific model of video card that allows all 3 to be used at once. I heard that AMD has such kind of capability. U may go to AMD site and search for the same.
Caution: Windows 7 has added support for multiple heterogeneous graphics cards from different vendors. But that doesn’t mean to combine ATI and NVIDIA cards in an SLI-configuration.