T3 respects that T2 did change the future. Judgement Day’s date moves and Skynet completely changes form. “Judgment Day is inevitable” refers to a general concept of a technological singularity where eventually some sort of tech will become self aware and decide to destroy humanity. But T3 does not assume all events are fixed and unchangeable. We later see different dates and forms of Skynet in The Sarah Connor Chronicles, Genisys, and Dark Fate.

“There is no fate but what we make” is Sarah’s mantra in T2. But she doesn’t change everything: she didn’t stop anything in the time loop from T1 and she didn’t change John’s fate in the future. John’s fate doesn’t change until the Sarah Connor Chronicles, Genisys, and Dark Fate, each of which gives him different futures. Really, the biggest thing Sarah and John do is continuously push the singularity date back further.