Want to use onboard graphics with the dedicated graphics card

tahseen

I've been using this desktop (Aspire AM5621-E5612A) for the last 7-8 years that came with ATI Radeon 3450 HD Graphics with 3 outputs (DVI, VGA, HDMI). This graphics card works fine with 2 monitors (1 DVI, 1 HDMI). But when I connect 3rd monitor to the VGA port, that monitor doesn't work. This card can serve only 2 monitors at a time. I tried to connect the new monitor to the onboard VGA output but that didn't work either. I checked the BIOS in the hope to change the graphics card setting but couldn't find any options. The BIOS is American Megatrends version. Any advice on how to use the 3rd monitor will be highly appreciated. Thanks in advance.