Think i figured it out. At least for me. Turns out that it is not just the "three finger gesture" that turns this on, for me the "one ear gesture" does too.
It ONLY happens to me when I am trying to cradle the phone between my ear and shoulder. What happens is the top of my ear moves far enough away from the proximity sensor to allow the screen to come on then my ear is on the touchscreen and the phone is interpreting this as the three finger swipe.
So this is my issue, but I could see on other people's phones that this could be the same, or it could be an indication of a proximity sensor issue.