I haven’t done everybody’s numbers but it seems to me that 3rd year qbs are inconsistent still. Mahomes may turn out to a huge, generational exception.
Another exception was Warner but then Warner’s 2nd year in the NFL (99 obviously) he was older and more experienced in comparison. His first training camp was in 94. By the time he was a starter in 99, he had been to 3 training camps, had been a graduate assistant for the Northern Iowa team, played in the Arena League, played in NFL Europe, and had been on an NFL roster for a year, plus was 28 years old. He wasn’t this 24 year old still maturing kid.
A more typical example is Roethlisberger. In his 3rd year, BR had 8 games with a qb rating below 80 (5 of which were below 60, including 2 in the 30s). In those games he had a completion percentage in the 50s 5 times, and also had only 5 TDs to 21 INTs.
Also, Brady. In his 3rd year, Brady had 8 games with a qb rating under 80 (3 of which were below 60). In those games he had a completion percentage in the 50s 5 times, and also had 6 TDs to 9 Ints. His last 3 games of his 3rd year, Brady’s avg. qb rating was 57.7 and he had 2 TDs and 3 INTs.
In fact Goff did better than that in his 3rd year. In 2018 (regular season) he had 4 games with a qb rating below 80 (2 of which were below 60). In those games he had a completion percentage in the 50s or lower 3 times, and also had 2 TDs to 7 INTs. Bad…but unlike BR & Brady, that’s just 4 games.