What is the 3-metre rule that was ignored during Rohit's controversial LBW dismissal?

MI captain Rohit Sharma was declared LBW out after RCB reviewed on-field umpire's decision. He was declared out despite his front foot being 3.7 metres away from stumps. The three-metre rule was apparently ignored. As per the rule, if distance between batter and stumps at point of impact is three metres or more, the decision will always be not out.