The only thing I can say is that is what those specific three manga are called because of their huge impact during that era, the same way I can say a burrito is not Italian food, it’s just not that, the term big 3 was coin specifically for those specific manga. Think of any term that’s been coined for something, baby boomer, i as a millennial can not say I’m a baby boomer if my parents had 10 kids, it doesn’t matter, or how about the MCU, if you are familiar, phase one ends with the Avengers, Iron Man 3 is phase 2, saying it takes place the same year why isn’t it phase one, it doesn’t matter, that is where the phase ends. If you don’t get the explanation you’re either trolling or a lost cause.
Edit: I also love how you quote only one part of my explanation and ignoring the part where I put it doesn’t matter what the best selling or anything else.