I am not quite sure what you mean by “i am using mini-hdmi to hdmi cables” but i assume you mean you are using mini-dp to hdmi cables.
When you want to use more than 2 screens you need to enable the Eyefinity Feature.
You do this by using the Displayport output on the Card (easy in this case).
OS needs to be WIN Vista or newer.
When connecting Screens with DP input, you can use all passive / normal cables (although we do advise to always use at least 1 active adapter cable).
When using screens without DP input, you will need Active Adapters for all screens after the first 2 …
Screen 1: with normal or passive Mini-DP to HDMI Cable (or adapter)
Screen 2: with normal or passive Mini-DP to HDMI Cable (or adapter)
Screen 3: with active MiniDP to HDMI Active Adapter with EF support and HDMI Cable
Screen 4: with active MiniDP to HDMI Active Adapter with EF support and HDMI Cable
Active Mini-DP to HDMI Adpater with EF support for instance: