The tailor who made my kilt told me that an individually fitted kilt only needs 1 strap each side. The reason for the extra strap, he said, was down to the rental market so that a variety of body shapes could be accommodated by adjusting the third strap. Another "one size fits all" solution by the kilt-hirers just like the white hose. No doubt "off the peg" kilts will also have three straps for the same reason.