I'm afraid you simply cannot run 3 monitors on that setup, especially not for gaming. Your video card (the Nvidia GT 520) will probably support 2 monitors, but definitely not 3. It MIGHT be possible to use the integrated video outputs (on the motherboard) simultaneously with the Nvidia video card to output to 3 monitors at the same time, but you will not be able to run games on that configuration, and I would not be able to help you to achieve that, if it is even possible.
A possible solution would be to upgrade the video card, as you asked first. You would then be able to output to 3 monitors simultaneously, but your computer is much too old to be able to play any demanding game on 3 monitors simultaneously. Moreover, with a 200$...