A R9 280 can support up to 6 screens.
The first 2 you can just connect directly with (passive) normal cables.
To be able to use a third screen you normally have to enable the Eyefinity feature.
You do this by using (one of) the DP output(s).
When using a Screen with DP input, you can also use a normal (passive) cable to connect from DP output to DP Screen. When using a screen without DP Input you will need an active adapter cable with eyefinity support for the third (and any after that).
So if you want to connect 1st Screen with HDMI, the 2nd screen with DVI and the 3rd with VGA, than you will need an Active Min/DP to VGA Adapter cable like this one CAC-1102