>So because this 20% chance doesnt come to fruition for 3 people (which doesnt even meet the minimum number required for a 20% chance to actually pay off once), it's automatically gender bias, rather than skills bias.
That is assuming that the selection is entirely random, or all are equally skilled and productive. If one of the female engineers where indeed better than their male colleagues, then this would bring her into contention for the role. That again is assuming that the promotion is solely based on their technical ability and ability to deliver. Itmay well be that the position that they will be promoted to will need other skills that they may not have exhibited.