There were alot of draft pundits that said that none of those 3 were NFL ready. Some even said none of them were really worthy of a #1 pick, and there was no definite #1. Trubisky only had one season as a starter, and didn't work much under center. There were doubts about Watson's abilities as a pocket passer, his accuracy, his mechanics, and whether he'd ever transition to the NFL.. Mahomes was considered a gun slinger that played in a trick offense, and that he was a system QB that wouldn't be able to repeat his gaudy numbers in the NFL.
A couple years later, lo and behold all 3 are productive in the NFL and still have room to grow. Watson has shown he can do it all. Mahomes lit it up in spite of coming from a gimmick offense. Trubisky shown that his ability to run in college translates to the pros, and that he has a strong arm. He still needs work, but the potential is there.
Trubs is the 3rd best, but that doesn't mean he can't end up as the best among them. He can run as well as Watson, throw on the run as well as Mahomes, and has similar if not better arm strength and accuracy. His issues are accuracy and mental mistakes, issues that can be overcome.