You can't connect 3 external monitors without some additional hardware.
A couple options:
(1) You could use a Matrox Triple-Head-2-Go [
http://www.newegg.com/Product/Product.aspx?Item=N82E16815106011&nm_mc=OTC-Froogle&cm_mmc=OTC-Froogle-_-Video+Devices+++TV+Tuners-_-matrox-_-15106011 ] to connect 2 or 3 monitors to the DVI port. It will support 3 monitors with up to 1680 x 1050 resolution (i.e. the native resolution for displays up to 22") or 2 monitors with up to 1920 x 1080 resolution (the native resolution of larger displays).
(2) You could add a couple of additional USB-based graphics adapters for the additional monitors, such as these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16812232019
