that the three powers (Saidin, Saidar, the True Power) must become one. I don't consider this very likely, but it could fit.

I'm not sure that Rand and Moridin will become as one, although it's possible. I predict that Rand's soul will be used as the final seal to the DO's prison, and it would only be fitting if Moridins were used as well somehow, and this might involve a merging.

I think that the three women merging into one is not only unlikely, but also a little absurd. It doesn't make much sense--there's nothing to indicate that they would merge, and plenty to indicate that they won't.

I'd say that, right now, the Rand/LTT/Moridin merger has the most evidence for it; Rand and LTT's threads are one in the same, and this thread has somehow been melded to Moridin's via the crossing of balefire.