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Marvel didn't really have a "Big Three". It has a Big One (Spider-Man), and a really big team (X-Men). At a distant third, you might have Fantastic Four but that was more contestible. Spider-Man and X-Men were far far ahead of the rest. Do you think it's an accident that when Marvel were optioning rights to bail out of bankruptcy, Spider-Man and X-Men were the ones that studios had most interest in?
Iron Man, Thor and Captain America were considered the "big three" of the Avengers which you know didn't have the sales, fame, influence that the X-Men did. Furthermore, calling it a "Big Three" is also a misnomer because it implies a level of equality between the three. Of the lot Iron Man was always the least prestigious of the bunch, there wasn't a single run in the classic era (and arguably still hasn't been) as good and important as Simonson's Thor, Englehart on Captain America, Roger Stern on Captain America not to mention the classic Kirby Thor and Kirby Cap issues. There have been above average Iron Man stories and runs (Michelinie, Fraction, Ellis briefly) but even those stories don't rate on the level of the best Thor, the best Cap...leave alone the best FF, X-Men, Spider-Man.
The 1000 people that Jim Shooter talked about didn't know them.
That was comics in the 2000s, no matter how much they sell, it doesn't come close to mainstream exposure and raising out of obscurity.
That happened because Marvel wasn't getting enough revenue from the movies since they sold the rights to Fox and Sony at "over-the-barrel" prices because they were desperate. The MCU originated for pure profit reasons not out of real love and affection for the Avengers over other properties. David Maisel, the godfather of the MCU, figured out that the characters they still had rights to would lead to the Avengers and if Marvel created its studios and made the movies themselves they get a bigger cut.
It showed that Marvel was doing all it can to maximize and generate interest out of all its properties, which has always been the priority for them, it was the reason for the shared universe in the first place, it was the reason they resorted to chicanery to make sure characters like Namor and the title of the Human Torch stayed with them and so on.
The fact that Marvel outsourced Avengers to Image studios indicates the lower level of prestige it had at the time compared to Spider-Man and X-Men.