I'm trying to figure out the progression of the Terminator timeline. It's pretty clear up until the end of T2. Afterwards, we have two Terminators in different timelines sent back by Skynet to influence the timeline- the TX (in T3) and Cromarty (in TSCC). Potentially either of these timelines could have happened first, causing the other- there's room in both for time travel development in the future. So does anyone have a definite answer as to which caused which, perhaps by hints of the past by people from the future?
There is no causality between the two, both happened in separate, independent timelines.
So they happen in separate timelines, and are not related to each other.
There has been some speculation that the two timelines can be forged together. Please bear in mind that this is pure speculation, and in no way canon evidence.
After the events of Terminator 3: Rise of the Machines the war starts between humans and machines. John becomes leader of the resistance. Skynet manage somehow to make advanced robots earlier than the previous timeline (maybe from the remaining parts of the T-X and T-850), which lead to creating the "Cameron"-model and using it on the battlefield. After the previous attempts of killing Sarah and John Connor (T, T2 and T3), the war still goes on, and it's getting tighter. Skynet now use all resources on the battlefield. In a last effort they send a T-888 (an older model) back to 1999 to kill John Connor. As a counter attack, the Resistance are able to catch a terminator from the battlefield (a "Cameron"-model) and send it back to protect John Connor. Sending terminators back before the previous timeline results in "erasing" the Terminator 3: Rise of the Machines-timeline completely (or, alternatively, creating a separate parallel timeline).
Josh Friedman, the Producer of The Sarah Connor Chronicles addressed this point in an interview with IGN in 2007.
The short version is that TSSC takes place in an alternate timeline which means that there's no continuity between it and T3.
IGN TV: Are you working within the continuity of the Terminator movies, specifically Terminator 3?
Friedman: We're gonna go off on our own. I think the thing about T3 is, obviously there was just no Sarah Connor and that's something the fans were never happy with. I don't even think the people who made T3 were happy with that. It just wasn't a choice of theirs. You know, Linda Hamilton was going through some things and didn't want to be a part of it. They had a script at one point I know; there was a T3 that had Sarah in it. I've never read it. I don't know anything about it. But I know that they did and they wanted to do that. They wanted it to be kind of her trilogy, and it never could be. So I think that my feeling was this TV series to me sort of serves as the third act to that trilogy - what could have happened had we followed that after T2. So I almost think of this as T3. To me it takes the place of T3.
But also I think that sort of in the spirit of Terminator, it's an alternate timeline. I know a lot of people get very worked up about the continuity and the canon and all that stuff. What I try to do is stay very, very true to the first two movies and then sort of take it from there. But always remain true to the spirit of everyone's intent and again, take some time with this woman and explore what maybe would have happened.