The issue is most likely your adapter. EyeFinity can be picky about which adapters work (not as much as in the past though). I've never heard of a DP to HDMI adapter being used in the setup, normally most people get the validated apapters that are DP to DVI then use a DVI to HDMI cable.
These seem to be the best type of adapters, I use to run this one before upgrading to monitors with displayport. When I received the adapter the box even had the ATI/EyeFinity logo on it to show it. http://www.newegg.com/Product/Product.aspx?Item=N82E16814999033