I would just repeat the 2nd monitor setup i.e. Mini DP to Hdmi Adapter -> Hdmi -> Hdmi port third monitor. Or, if your gpu came with a dvi/hdmi adapter (some do) you could use that to connect your 3rd monitor as you already have one connected via mini dp. Only one monitor needs to be connected via dp to enable eyefinity.
Is the mini dp/hdmi adapter active or passive? Reason I ask is that one of the monitors must be connected via an active adapter as it needs the extra power for the added pipelines to hit the higher resolutions, passive will not work for eyefinity. Unfortunately , I have little experience with hdmi, all my monitors are connected via dvi, with one via active dp/dvi adapter. But if you are unable to source an active dp/hdmi adapter, you could try for the 2nd active mini dp/dvi adapter>>dvi cable>>dvi/hdmi adapter>>monitor hdmi.
No the adapters were both passive, which was my reasoning for using all caps on my explanation earlier when I wrote ACTIVE to point out the both my possible solutions I came up with used active converters.
I decided on ordering the Mini DP to HDMI active adapter even though it had no reviews as it was probably a new item. I assume it will work hopefully seeing that it is active, but we will just have to see when it comes in.