Hi there and welcome
Even if you have both HDMI & VGA outputs, you'll only be able to use one at a time. To get around this you need to use a USB to VGA adapter which will give you the initial monitor (as I advise to use the current HDMI port)
This is such an adapter that will work and will allow you the third screen.
If you would like any additional information or assistance, please do not hesitate to let me know.
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Let me know if you need anything else and please remember the service rating via one of the stars at the top then submit