I need to add a 3rd monitor to a Inspiron 3847. This will just be showing a timer/clock so I don't need high power graphics card. What I don't know is if I add a video card in the system if it will disable the onboard graphics card. Considering a USB to VGA adapter, will that work?
Are you saying you don't have any add-in video card now, and only use onboard Intel HD graphics?
On most modern PCs, when you install an add-in video card the onboard ports are disabled. But look in BIOS setup (reboot and immediatelypress F2) for a "multi display" option. If you have that option and enable it with an add-in video card installed, you will be able to use onboard Intel graphics as long as at least one monitor is connected to the add-in card.
Decent add-in video cards typically support 2 monitors so #3 could be attached to an onboard video port in your situation.
If you don't have the "multi display" option, you could either get an add-in video card that supports 3 monitors, or one that supports 2 and use a USB>VGA adapter for #3..