All you have to do is turn the big ring limit screw on the FD in (CW) enough to prevent the shift to the big ring. The proper setup of a Campy triple FD require 7 clicks of the finger lever. It requires one full sweep (5 clicks) to shift from the little ring to the middle ring, but it then requires an immediate click of the thumb button to recenter the FD cage, since Campy's mechanism has no built-in overtravel and return. Without the thumb button click, the left side of the cage will rub the chain. If any more clicks are used (12 are available), it means that the FD cable does not have enough tension and you're wasting clicks taking up cable slack.