It's a relatively new thing, at the very least. I haven't read every Avengers comic ever or anything, but I don't recall the Big Three Avengers ever really being "the Big Three Avengers" before "Standoff," the aforementioned arc from about 10 years ago by Johns, Grell, Jurgens, and Davis. They always had strong bonds between the three of them, and they were always prominent members of the Avengers just because they filled such important roles on the team--Cap as the leader, Thor as the top powerhouse, Iron Man as the financier/main science-type--but I don't think we've seen the level of reverence that they're receiving now. They were always great to have around, but they were never regarded as so intensely integral to the makeup of the Avengers before.