I understand that it was originally 3 night take-offs and landings in that type/class of aeroplane in the previous 90 days to carry passengers at night in the same type/class, but certain airlines pointed out that this would have been impossible at certain times of the year - e.g. transatlantic fights taking off in the summer evening and arriving in Europe the following morning after a period of night flying....... So it was changed to the current rule.