"When I Plug in the third monitor to the original monitor slot in that came with the computer ..." which I assume means you have an additional VGA or DVI connection on the motherboard itself. If this is the case go into the BIOS and see if it was disabled when the GeForce 8400 was put into the system. If so enable it as the secondary monitor and give it another try.
I have a PowerSpec computer and I am running windows 7. I currently have two monitors coming from an GeForce 8400 graphics card. I want to add a third monitor. When I Plug in the third monitor to the original monitor slot in that came with the computer it does not get recognized. Any advice on how to do this?

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