... that the simplest explanation is that as long as you use both saidin and saidar together, you're okay and the Dark One won't be able to taint either one. The female Aes Sedai refused to help last time, so it was only saidin, and so it failed. A theme throughout the entire series has been gender relations, and I see no better way to show men and women working both metaphorically and literally together to save the world.

Thus, the three becoming one would refer to two female channelers and Rand working together to use Callandor, the women directing the flows, to seal the prison. Most likely Rand, Nynaeve, and Egwene, the trio from the Two Rivers.

It seems to me that anything else would just be needlessly complicating the matter ...