I'm having a very similar problem with a 6950
2 IBM panels connected to the DVI ports and they work fine. To get the Chimei 221D working with them I got a mini-DP adapter.
Catalyst will let me select any 2 from the 3 but insists that one of the 3 must be deactivated. Cannot get all 3 working at the same time.
Sorry to perhaps state the obvious, but again: Are you really sure this is an ACTIVE DP to DVI adapter?
The way this works, as I understand it, is that these cards can generate only 2 TMDS (See Wikipedia) signals at once, which is what DVI needs. When you connect a DVI monitor to a DP port through a PASSIVE adapter (which simply connects certain pins on the DP side to certain pins on the DVI side), the card must still generate a TMDS signal, which is then simply routed through the DP port. An active adapter, on the other hand, accepts a true DP (Non-TMDS) signal, which is then actively converted to a TMDS-type signal BY THE ADAPTER. This leaves the 2 TMDS channels on the card free for use with the DVI ports.
Or something like this anyway.
So when you can only use any 2 out of your 3 monitors, that is highly suggestive of your DP-DVI adapter being of the passive kind.
Apparently, passive DP to VGA adapters DO work. On the other hand, the Sapphire adapter I mentioned earlier is pretty inexpensive as well (I got it for 25 euros), and that is definitely active.
Jeroen
Jeroen