If that third monitor is the only one that uses VGA, you need to get this converter http://tinyurl.com/nzx3etd
and use it on the DVI-I port of the graphics card so that you can connect it to that monitor with a VGA cable. The monitor that was previously using the DVI-I port should be able to use the DVI-D port.
If you have two monitors that can only accept VGA, one of them will require an active converter like this one http://tinyurl.com/kbcyc6m
so that the digital-only signal from the DVI-D port can be converted into analog signal that VGA requires.