I tried putting a third screen using the hdmi port but its on one side. Not center. And my hdmi cable wont fit. Also. Would the hdmi port become the main one. Or is it just for an hd tv
I do think that there are 2 different sizes of HDMI ports. A regular and a mini HDMI port.
In my experience on Windows 7, it tries to default your main screen to the one with the best resolution. This may or may not be your hi-def screen as there are different hi-def "levels" if you will (720, and 1080 I think, but it's been awhile since I looked at all the numbers) and it would just depend on what your other monitors are at compared to the TV.
You can change those settings depending on how your monitors are configured, it doesn't have to be in relation to how the plugs are placed.