Pretty sure Salvation is a direct followup to T3: the film explicitly states that as of 2003, Judgment Day had not yet occurred when T2 firmly establishes that Judgment Day originally takes place August 29, 1997. Salvation follows the "Judgment Day postponed due to the events of T2 timeline" that's established in T3. I assumed T3 takes place in 2003 but it may have been 2004
Oh and Salvation also goes out of the way to mention the nuclear fuel cells of the 800 series Terminators, something that was only established in T3.
Double edit: had to go back and watch again but when Marcus signs the consent form to donate his body for research, the header on the form reads "Cyberdyne Systems" which should no longer exist in the altered timeline (T3 establishes that Cyberdyne was bough out and folded into Cyber Research Systems). So it's possible that Salvation takes place in a wholly new timeline or someone on the Salvation staff didnt do their research into the canon
Also, Kate Brewster and the presence of T1's are carried from T3 into Salvation