I assume you mean the fastest setting (furthest to the right). I just tested here and it works fine. Note that setting it to the fastest setting means the double "untouch" must happen in 150ms and is fairly difficult to do (the next one from furthest right is easier at 200ms).
In that scenario, it will initially scroll, but then execute the 3-finger gesture provided you move far enough with 3-fingers. The three fingers do not have to come down at the same time. If there are two fingers moving at first, it is scrolling, then later if the third finger comes down it moves into the code that does 3-finger gesture.
But this trackpad has trouble distinguishing 3-fingers from 2-fingers (and 2-fingers from 1-finger) when the fingers are very close together. And that is more likely what you're running into. It is something at the firmware level that I can't do anything about.
