The integrated GPU is disabled when an add-in GPU, such as your GT230, is installed.
Your options are to buy a GPU that supports 3 monitors simultaneously or use one of the new USB video cards to run that 3rd monitor ( dont except to game with one of these. Fine for business graphics)
If you opt for the new GPU option, be sure the card you get has the proper outputs to connect all your monitors.
Thanks Popatim. I went with the new USB video card and am happy with it.