The two clockwise spindles are using one side of the belt, and the anticlockwise one uses the other side, so both sides are being used. I've never seen a double sided V belt, though that doesn't mean they are rare. Hope you can get replacements here though, or it would be best not to spend a lot of time finding a use for that deck. Since they only have half as much surface area as a single sided belt, they probably won't last all that long. Note the extreme measures taken in the design to maximise the wrap angles the belt has around the three cutter spindles: this is to try to minimise slip and therefore wear on belt and pulleys.
