Is it not possible that the three discs (per site) serve a double purpose?
1) Perhaps it is intended that we are to find the discs and to analyze them to decode the message(s) imprinted upon them
2) In that the discs are made from three distinct metals it should be possible from a hovering craft to then exactly pinpoint the location of the site and then re-apply the graphic (using radiated energy from the craft) exactly as it had been each previous time.
In other words for #2, they can be thought of as survey markers. In astronomy and other disciplines, "three-point alignment" is the minimal method of assuring that your map and reality are in sync.
If you wanted to return to Earth and draw a map in the grass every, say, 3600 years and wanted to do it in exactly the same place every time, placing three survey markers would be the way to go. In fact, using distinct metallic signatures for the three markers would help you to differentiate your actual site from the possibly many non-site hits that you could get while flying over and scanning.
It seems clear to me that the crop circle site as shown is likely not a hoax (given the cost to manufacture the discs). I would guess that the likeliest message that's encoded within the discs is the location of the "authors" if you will. If you review the external plates on our own Pioneer and Voyager spacecrafts you'll see that we mostly wanted to convey our location to others in similar fashion.