The three straps are a hold over from the high waisted military kilts. The top of the kilt was high just under the rib cage and had two straps at the waist. The third was lower down on the hip to better close the kilt and hold its shape. More recent kilts are not a high up on the waist, often just above the navel and therefore the third (hip) strap is not really needed. If you want one it could be added. Try the kilt if it fits and you like it wear it.