I don't think it works like this... You can't count on these guys coming out on a schedule like this. They could always go back to school. In fact its been rumored that Tua may decide to not enter the draft next year. Remember when Matt Leinart was going to be the consensus number one pick but decided to stay in school and then got drafted 10th or so the next year?
Bottomline, if you are in position to draft a top QB prospect in the draft, you do it until you have a bonafide QB on the roster. So if Kyler Murrary drops to 13 this year, I think you draft him. If he fails year 1 and you are sitting there with a Top 3 pick, you draft Tua/Fromm/Herbert if they are there, and so forth until this QB situation is sorted out.
Some things are more likely than others though. The entire base upon which Saban has built his dynasty at Alabama has been that he will bet prepare you for the NFL, and if you're good enough, showcase you to the NFL. He follows through on that with his players, and thus both the players and the staff are comfortable letting their talent go early. In fact, they recruit based on that expectation, and use those earlier successes to fuel their future recruiting, which is top notch every year.
So Tua is a guy I think we can be confident will come out in 2020. Likewise, I think that Justin Herbert came back to college for 1 year, so he's someone we can count on being there for 2020 as well. The only one we aren't quite sure about is Jake Fromm, but I'll be quite surprised if he doesn't come out in 2020.
Now, Trevor Lawrence, that kid has his head screwed on well. He has embraced the family element of Clemson, a school that works like a family not a factory, so there's a possibility Lawrence, who is ready for the NFL today, may actually not come out in 2021 … but he's already so far advanced that I see him coming out early as well.
I recommended Justin Herbert should return for another season, because he lacks the accuracy to succeed to his level of talent in the NFL. If he works on his footwork, read and react, and precision throwing, he should improve that accuracy and cement his top 10 draft position.