- Experiment a bit with flag settings, like here:
https://en.bitcoin.it/wiki/Mining_hardware_comparison

- Stupid question, but are you saving the settings? go to GPU-Z and check the clocks when GPUs are under load.

- Does it only happen with only all three GPUs connected simultaneously? How does it look with only one GPU connected? Same mhash/s? (if it's higher then there might be a problem with your PSU)