The Military uses of the Palantiri.

[Alfirin]

I have no textual basis for this, but I've always though of the Amon Sul stone as being sort of a "clearing house" for the northern stones, that is, if a message needed to be sent from the northern stones to the southern ones, it had to go through the Amon Sul stone. It's mentioned in the UT that the size of the stones has an effect on how far they can see/communicate; it's possible that, given thieir size, the other two just didn't have the range to send a message to the southern stones on thier own. (Yes, I know that, technincally, the Elendil stone (the one that looks only West), which was "normal" sized, had an even longer distance to look, but its fairly obvious that that one is "special" and may be made differently than the others) The Orthanc "master stone" is likey large enough to allow its range to extend from one end of Kingdoms to the other, but the "ordinary" ones may not be.

A question of my own whne they say that the Orthanc stone could see/and evesdrop "all of the others" does that incude or exclude the Elendil stone, seem to me that evesdropping on a stone that can only look in one direction (and a direction that it was unlikey trouble could come from) would be all but pointless.