For dwellings of three or more dwelling units, there shall be two side yards of a total width of not less than 30 feet, plus four feet additional for each dwelling unit in excess of three contained in the building. The width of the narrower of the two side yards shall be not less than 1/3 of the total width of the two side yards. However, any side yard width in excess of 60 feet, accumulated under the above schedule, may be applied as additional widths for one or more outer courts which, to start with, are not less than 36 feet wide and 38 feet deep.