There are alot of prophecies out there that are expected to be fulfilled in ToM.

 

Bear in mind that BS has said that there are going to be some number of prophecies that will remain unfulfilled by the time the last book is written and the series ends.  Yes, some have to be fulfilled, but we may never know just what the "blade of light, three are one" refers to if it refers to something after the last battle.

 

I would imagine the Blade of Light would be someone using Callandor with 2 women to guide it. What blade shines like a beacon, when being used. Callandor of course. Whether it'll be Rand using it is still suspect.

I'm may have read ahead but Avienda and Elane and Rand are "one" ...along with Min but as she cant channel she wont be part of the 3 are 1 with a woman leading the channeling.