Install the latest catalyst suite from AMD. Then go into the catalyst control center and check that it can see all three displays. There should be an 'identify screens' option. Configure them as an eyefinity group.
If you want to test your adapter to see if it is working, then shut down the system, disconnect the other two monitors and boot with only the single monitor attached via the DP -> DVI adapter. It should work fine.
Also, what resolution are your monitors specifically?