Okay. Let's analyze this using information from all three games released. The secret ending in this game shows another Kingdom Hearts game in progress. This can be supported by the fact that Kingdom Hearts itself was never truly destroyed. As long as darkness lingers in the hearts of people, Heartless will form. Keyblade weilders use their blades to destroy them, and their released hearts re used to fill Kingdom Hearts again.
"Master of Keyblade"
In Kingdom Hearts, we learn that Sora is the Keyblade Master. However, we also notice in that game that Riku is also a weilder. Kingdom Hearts II revealed King Mickey was also a weilder, as was Kairi (thanks to Riku).
"The Lost Two"
It would appear in Kingdom Hearts that the lost two would be King Mickey and Riku. Kingodm Hearts II revealed at the end Sora and Riku stating that they "would be the darkness", but eventually ended up back at Destiny Islands. This could leave Namine' and Roxas (who are the "Nobodies" of Sora and Kairi) to be the lost two, as they are lost inside their creators. This secret ending could apply to the two characters who join the prodominantly male character in the beginning.
"Memory of Xehanort"
Xehanort's memories are captured in the Ansem Reports stated in Kingdom Hearts. This could also apply the Ansem the Wise's reports. Also, Nobodies linger on the edge of nothingness, which was one thing Xemnas said before the final battle. Somethign he said migt have applied to this, and thus be a memory of Xehanort.
"Chasers"
Those who chase. My guess is someone who seeks out Keyblades for their own use. No evidence from any game can suipport this.
"Keyblade War"
A war involving Keyblades. What we see here is the ruins of thousands of Keyblades. Since one person can only weild one Keyblade, thousands of weilders had to have been involved. This could either be one of two things; this game takes place in the future or past. In the past, there could have been many Keyblade Masters. However, it was dwindled down to three at the end of the war, and thus these three combined formed a sort of trinity. If it takes place in the future, then these Keybladers could have spawned from the original three. Riku gave a keyblade to Kairi, which she can use without it warping back to Riku. Perhaps this war involved some more spawned Keybladers, and Riku, Sora, and the King were defeated. THis could be why their keyblades are left over. Perhaps those three are chasers, after their keyblades.
"It all began with...birth by sleep."
Sora was put to sleep in Kingdom Hearts CoM. Roxas could have been created during the time of his sleep, though this can't be supported since Sora was turned into a Heartless in Kingdom Hearts. However, it doesn't state in any game how long it takes for a Nobody to form.
So...what do you think of my theories? BTW, the three character in the beginning have been rumored to be Final Fantasy XII characters. I don't know if this is right or not.