Big three - in what sense?
Your website says Iguazu Falls, Victoria Falls, and Niagara Falls are the biggest legitimate waterfalls. What do you mean by "biggest"? There are plenty of waterfalls that are taller than those three (Angel Falls is over nine times taller than any of them). Those three are among the widest, but there are a few others that are wider (like the Chutes de Khone, Salto Pará, Kongou Falls, and Saltos del Mocona). Lots of websites seem to think those three are the most impressive, but they don't seem to have a lot of metrics. Are those the three widest waterfalls with tall vertical drops? Is that what makes them so impressive?