Kevin and JustNick,
Sorry for coming to this party late! But, I have done a lot of research on this issue, and I believe you cannot set up three monitors with HD 4600 with only HDMI and DVI enabled monitors. You must use VGA on one of the three monitors, although you can get 1920x1200 resolution even with the VGA input.
Please let me know if I'm wrong because I have two DVI monitors and one HDMI monitor.
My motherboard (Asus Z97-A) has one DP, one HDMI port and one VGA port.
I connected one DVI monitor to the DP using a DP to DVI converter. I connected the HDMI monitor to the HDMI port with a straight HDMI cable. And, I connected the second DVI monitor to the DVI port using a straight DVI cable. I could get two monitors working at the same time in various configurations, but I could not all three to work at the same time. I guess that when I connect the DVI monitor to the DP via an adapter, the HD 4600 still recognizes it as a DVI monitor. And, the HD 4600 does not allow a DVI, DVI and HDMI combination. So, I think my only option is to remove one of the DVI connections and instead connect one of the monitors via VGA.
Again, please let me know if I'm wrong or if I am missing something.