Not sure about how to disable Crossfire, but! I do remember I needed to disable ULPS (Ultra-Low Power State), what it basically does is disable cards that are not in use, so by disabling ULPS you make sure that all cards work at all times.
To disable ULPS:
1. Go to the registry ("regedit" in windows search - make sure you open it as admin).
2. Make sure "Computer" is highlighted and than go to Edit -> Find -> EnableULPS
3. Double click on EnableULPS on the right side and change its value to 1 (Hexadecimal base).
Restart PC and check to see that the registry was saved (again, make sure you access regedit as administrator).
value of 1 would be on. i set it to 0 and i'll try it out in a bit. hopefully this works
If all 3 cards fit (they do on my mobo) did you try putting all 3 directly onto the mobo? (temporarily of course)
i dont think all 3 cards fit. haven't tried since it would be unrealistic to use 3 290's all unpowered (or at least that is my assumption)