That's just not true. During the era of the "big 3" Naruto, Bleach, and One Piece were not consistently the top 3 selling manga in Japan and internationally. During much of the era of the Big 3 Bleach was in decline, and Naruto never really sold quite as well as One Piece. There were several series even within Jump that outsold Bleach or Naruto during the era of the Big 3.

The Big 3 is a largely western perspective concept of the 3 biggest manga series to cross over into the west during this particular era in Shonen Jump. To show even further how the "big 3" is a concept pretty disconnected from sales figures recall that during that era Naruto and even Bleach we typically viewed more prominently than One Piece although One Piece was always more popular in Japan in the west Naruto and at times Bleach was held in higher popular regard.