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#11
I checked the model of your PC at the Dell website, and it says that it does support a dual onboard/graphics card split. So you could theoretically get this working. I wonder if it's a setting in the bios?
EDIT: The information I can find on the Dell forums says that with this PC, plugging in a secondary graphics card turns off the onboard graphics. Here's a link to a relevant thread (Warning, it's confusing):
Optiplex 755 and two monitors - Desktop General Hardware Forum - Desktop - Dell Community
So I guess the short answer is no, your onboard graphics port is not going to let you hook up a third monitor on this particular PC.