i dont know its already discussed here or not but thats my small wish
so guys we all know uncharted 3 left so many question marks????? in its story like we never figured out what happened to chloe and cutter after that bus cutscene?, what chloe and cutter were doing in syria before the arrival of drake and sully?, how sully got kidnapped by talbots army?, what sully was doing when drake drank the water from fountain in lost city?
how about naughtydog makes a single player dlc containing 3-4 chapters just like lost in nighmares and desperate escape dlcs of resident evil 5
The things you have asked are explained, but only in passing. They don't really dwell on the details too much in the storytelling, but they are all there, mostly.
Cutter obviously left because he broke his leg. As for Chloe, she does say to Drake that everything in Syria was too close and that it just isn't worth it. She doesn't go along with Drake's obsession so she leaves.
Before Drake and Sully arrived in Syria, Chloe and Cutter were looking for the hidden crypts, to find the knight's tomb (just like Drake and Sully were in the chateau). Drake and Sully were never actually meant to go to Syria, but they thought that Cutter and Chloe were followed and possibly captured, so Drake and Sully went to find them.
When Drake and Marlowe are talking in Yemen, she says she needs Sully as he has the co-ordinates of the lost city, and then Talbot informs her that they have located Sully in the city. Sully is unarmed and, as you say, Talbot has an army, so it makes sense that he was captured.
I can't really say what Sully was doing exactly. Presumably fighting bad guys while trying to catch up with Drake.
I would love single player dlc, but they have said that it's not going to happen. I believe thay've said that it's because, if they're going to start working on something, they'll constantly want to increase the scale of it to make it bigger and better, so they may as well just make a new game. Hopefully they'll do single player dlc with Uncharted 4.