-The first two monitors can connect to the graphics card with any display output on your product: HDMI, VGA, DVI or DisplayPort.
-The third (or greater) display must be connected to the graphics card via DisplayPort.
-If your monitor does not have a DisplayPort connection, you will need an active DisplayPort adapter for it.
-Every family of GPUs supports a different maximum number of displays.
Based on this, I would connect one monitor via VGA, one via HDMI, and not use DisplayPort at all.
1x Mini-Displayport to DVI adapter (Must be an active adapter) then from there a DVI to HDMI Cable
1x DVI to HDMI Cable
1x HDMI to HDMI
I would completely avoid VGA all together, it usually causes issues in EyeFinity. VGA isnt a digital signal so adding the converter in to convert to VGA doesnt seem to work well. Since your monitors only have HDMI, the best solution for you is going to be DVI to HDMI cables. This leaves the signal digital.
You really didnt need to switch monitors. The HDMI inputs would have worked fine, its just a matter of getting the right cables. HDMI and DVI are pretty much the same accept the connection (and HDMI carries sound). Anytime you have one plug you can always use an HDMI to DVI cable. Even if you get monitors with DVI instead of HDMI you would still then need to use the same cable.
If you are getting new monitors and want to do things the easiest way possible get monitors that have displayport. When I upgraded monitors in February I got 3 asus VE248Q monitors that have HDMI and Displayport. I run 2 using displayport (Mini Displayport to Displayport Cables from monoprice rather than a mini to full displayport size adapter), and the third using HDMI. I also run one of my old screens as a 4th (3+1 setup) using DVI.