47.17.For the purposes of sections 47.14 to 47.16, the by-laws of the brokerage permit holder may provide that the trucks of a group of subscribers who are related corporations within the meaning of the Taxation Act (chapter I-3) will be classified as if they belonged to a single subscriber, and that the group may designate, as first trucks of the group of related corporations, the number of trucks determined in the by-laws of the holder, without exceeding three.