you HAVE to use a displayport for any monitor connected beyond the 2nd, no matter the configuration of the first 2 monitors. Examples:
I have one VGA, one DVI, the 3rd must use displayport
I have one HDMI, one DVI, the 3rd must use displayport
I have one DVI, one DVI, the 3rd must use displayport.
Hope this helps! REMEMBER, you have to use an ACTIVE displayport adapter, not a passive one. I can't stress that enough.