My computer's graphics card can connect to a monitor in three ways. It has two dvi ports and one hdmi port. Is it possible to connect two monitors via dvi and another monitor via hdmi resulting in having three monitors or can my graphics card not handle it? My computer is running Windows 7 Home Premium 64 bit. My computer is at this address so you know what I'm talking about http://www.newegg.com/Product/Product.aspx?Item=N82E16883113152
Actually, you need an eyefinity card to run three monitors for the same application. As in a computer game running on three screens, with the image streched across all three. Or are you just talking about wanting to hook up three monitors for various Windows applications? That's different, and way more doable, and way less costly.
Three monitors will work fine as standard. You only need eyefinity to use three monitors for the same fullscreen program at once. A desktop-based program like excel can still be dragged across three monitors without needing eyefinity. Even if you did need eyefinity, the HD5850 in that system supports it [though an active displayport dongle is required]
I've read online that the hdmi port is part of one of the dvi ports and plugging in a device either into the hdmi port or the dvi port will act as if both ports are used. I think if I wanted to do this I would need one of those usb display adapters. Is this true? If so what are the disadvantages with usb display adapters?
You're quite right, didn't know that. The HDMI port is indeed a conversion of one of the other DVI ports, so to use three monitors you will need one of them to be run on displayport (However, this does not mean you need to use eyefinity). You may be able to get a displayport - DVI or HDMI adapter I suspect, as not that many monitors have displayport connectors. USB display adapters work, but only for the bare basics. They cannot render anything, only produce a desktop image, so they won't run aero, they probably won't play video, and they certainly won't play any games or animations.
Someone in my office uses a USB display adapter for her third monitor. It works well. But these are mostly office applications. I know that she looks at youtube and things like that and has never complained. If you want to use the device for productivity, I think it would be fine. But for a multimedia or gaming solution, maybe not.
From what I've been told and from what I've tried you need to have a displayport port and an lcd that has a native display port. An adapter will work for the display, but will not work when trying to do a 3 screen setup.