One idea might be to use Callandor to bait Shaidar Haran to use it to break Shai'tan's prison. My guess is that Shai'tan might try to enter Creation (ie: break his prison) by passing through Callandor. Perhaps he would have to release his grip on the Bore to do that, and the interaction of the three ta'veren being One might give enough to the Wheel to "heal reality", that is to reknit the Pattern without this hole in it. The way to destroy Callandor, where part of Shai'tan would be trapped, would be the SL taint, ie: Fain. He and the sword would annihilitate one another, as happened with Mashadar and the Taint in WH.

There are tons of possibilities, really, but it seems to me almost certain that if Callandor isn't a red herring and will be used at SG, it won't be to provide Rand with a powerful circle of three.