No, they can be different brands. However Surround (2D) mode requires that all three monitors are set to a common resolution, refresh rate, and sync polarity.
For example, you can use two 1680x1050 monitors on the sides and a 1920x1080 monitor as your center display, as long as all three monitors are set to the same resolution (likely 1680x1050 in this example).
NOTE: This is true for 2D Surround mode only! 3D Vision Surround mode requires three identical brand and model displays.