For eyefinity, the 3rd monitor must be connected via displayport in order to achieve the higher resolutions enabled in eyefinity. If your monitor does not support displayport then it must be connected via an ACTIVE adaptor, not passive. The following link has a list of approved dongles.http://support.amd.com/us/eyefinity/Pages/eyefinity-dongles.aspx
If you're already using an active adaptor, you then need to create an eyefinity group in the catalyst control centre. Instructions found on this link.http://www.tomshardware.com/forum/344343-33-eyefinity-setup-guide
If your monitor is displayport capable, it needs to be enabled in CCC (I think, sorry, not sure about this but read in another thread that this needs to be done)
Hope it helps, good luck.