The "three in one" thing...
Per Cadsuane, we know that Callandor can only be used safely in a circle, which means women have to be involved.
Depending on when Rand's confrontation with the DO takes place, Elayne may not be available. I figure she is maybe five or six months along now. Far enough that she can't reliably channel, anyway. So she's at least three or four months from being able to actively participate in anything OP-ish, and I don't know that there's three or four months left, Rand-land time, until Tarmon Gai'don.
That leaves Avi, Nynaeve, Alivia, Egwene... or, maybe, Cadsuane to make a circle with Rand. From a strength standpoint, anyway...
Aviendha... I can't imagine that she would let him go and face something like that without harking back to her Maiden days, picking up a spear (figuratively speaking) and going with him. Nynaeve... will she be with Rand, or will she follow her heart and go to Lan? My heart says she'll go to Lan and Tarwin's Gap. Alivia will probably be there with him - if you consider that his "death" might be a result of that fight. Egwene, I think, will have her hands full in TV; she isn't out of danger yet. The truth about Moggie will come to light at some point; don't think for one second that Lelaine and Romanda wouldn't love to have that little bit of info. And that's just the political danger! Remember that political danger in the White Tower can cost an Amyrlin dearly - as in her head. As for Cads... I don't see her surviving TG, and what better way for a legendary Green to go than in the most critical battle in the history of mankind, yeah?
So... if I had to hazard a guess as to who the "three" might be, I would say Rand, Avi and Alivia or Rand, Avi and Cads.
On the other hand, the "three as one" thing could refer to the three ta'veren. Rand with Callandor (and a circle) battling the DO with the Power... Mat leading the armies of men in a more corporeal fight... Perrin (and the wolves) fighting the DO in T'a'R...
Just a thought.