RAW has already been mentioned. However, I agree RAW is weird in this case.
If I would run a game I would go with one of these option.
Alt 1. The opponent has a facing, and the "flankers" in front of the opponent will not get a bonus unless having the Gang Up Feat.
Alt 2. If an opponent is flanked, everyone fighting it gets the bonus. The Gang Up Feat is for when 3+ are fighting one opponent but none are in a flanking position (for example, standing in line).
X
XO
X
or
XO
XX
