I take it you are referring to display output ports. At least one of those back ports will be DisplayPort. If your monitor does not natively support DisplayPort, you will need an active adapter in order to get the display to work when plugged into DisplayPort
If your card has 2 DVI and 1 HDMI port, you can't use all three of those ports. AMD cards will only allow 2 DVI, or 1 DVI and 1 HDMI port to be used at a time. All additional displays must be run through DisplayPort. If you have only one DisplayPort, it is possible to connect additional displays to that port through daisy chaining, though that will require additional adapters if none of your displays have DisplayPort connections.