I suppose it might be possible to use it as triplizer, but that's not what that is. That Simplex adapter is for mounting 3-pin chain rings to a 5-pin 50.4 mm BCD ring.

I think the easiest (and perhaps only) solution for you will be to find a freewheel cog like the one Ben showed a couple posts back; drill three holes in that and three holes directly in the arms of your crank; and mount it with a bunch of spacers.