A constant with Jordan is that he always twisted things around rather than mirroring myth. There's tons of characters inspired by Arthuriana, but Jordan shifted everything around. The character named after Guinevere (Egwene), is Queen of Tar Valon and a "fairy". Rand is shared by two more "fairies" and Min, who is a prophetess, so a kind of magician too. The motif of the dead King being brought to the Fairy Queen of Avalon by a bunch of fairies is there, but their relationship to the King isn't from "classic" (late medieval and christianized) Arthuriana. As you mentionned (though not in this sense), Jordan went backward from there. The celtic myths that inspired this aspect of Arthuriana had these women as three "wives" of the dead hero (and in Irish myth specifically it gets more complex because the three women are actually aspects/avatars of a single female goddess and three different goddesses at once).

I'm not sure Jordan worked this "backward" in the sense you mean, though. It's as plausible that he decided on the triad of wives first, and later on settled that it's those three who would be in the Arthurian "dead king" boat scene. Strictly speaking, in classical Arthurian things, these three women would have been more properly played by Moiraine/Cadsuane/Nynaeve who variously played his three fairy helpers yet somewhat antagonists through the story. In quite a few versions, the King's Queen is dead and in the boat going to Avalon as well.

As for a fourth women, we'll see... this isn't because it's the three women with Rand who are important that they are necessarily alone in the boat with Rand. Many have theorized that there won't be any "real" boat, that the boat is actually Egwene's Skimming platform. If so, she'll be the fourth, but her role will be more remote. Egwene may well have had a dream which heralds/completes this scene, the one where she sees a man on what seems to be a funeral cot in a tent, and this man (perhaps already in a shroud, she didn't say) isn't dead and it's important that he doesn't die, but outside the funeral pyre is already being built and some people are already cheering the man's death while others cry in mourning. It sounds like Egwene might be the one of the group who will know Rand isn't really dead (though Aviendha ought to remember the WO's dreams she relayed to Rand, while Min's viewing's interpretation suggests she believes Rand in the bier is dead, and the three women around the bier are mourning him).

We'll see. That scene could very well happen at the climax of the next book (more of Jordan twisting things around, the Arthurian Cyle ending there), after some mega Arthurian armaggedon at Caemlyn (Camlann, Camelot). We know the characters are set for a reunion at the "event" which starts the LB proper. It could well be heading for something like this, as there's nothing much Arthurian in what we can puzzle out for the LB -sealing of the Bore, the Horn sounded etc. That is inspired by other sources altogether, and it may be that Jordan planned to "wrap" the Arthurian stuff in Caemlyn as TG starts. Rand's epiphany in TGS was "surprising", in the sense it comes very early in the finale. It could well be that the Light is heading for a major road bump soon after (timeline-wise, so at the end of TOM), and that "new Rand" will be "dead" and out of commission for a while (ie: healing), to return to the world in the nick of things further down the line, for the finale of the Last Battle, or to begin to turn the tide midway into it.