It's way to early to determine this, really have no clue how teams really are until around game 3.
But since we're bored and waiting for the season....
1 - Bama - The talk is about the lack of returning players. But we play AT Bama, and DeBoer has them going in the right direction (9-4 in his first season, then 11-4 with an appearance in both the SEC title game and CFP.
2 - OU - Again, on the road. Seems people are thinking OU will be better than Bama, but I usually lean toward what happened last year when the two teams seem to be a "push." Bama beat OU in the CFP. Also, it's later in the season for us. I think home field advantage gets more currency deeper into the season (e.g., injuries and second and third string players may be more comfortable playing at home, etc.).
3 - Texas - As is typical, Texas gets overhyped every year. That said, they are stacked. But I think home field this year at night makes this easier for us than going on the road to Bama and OU. Both teams will have injuries, so that's of course the wildcard that makes these kind of prognostications really just fun offseason day dreaming. But sitting here today, just as injuries partly affected us last year (Reed ankle, Moss out), late season games favor the home team.
Again, the above is how I see as OUR most difficult games, NOT where I think the above teams may end up relative to each other in the rankings, as I think the original question asked.
If we're saying of the three, where will they likely be ranked relative to the other by the end of the season, Texas has the toughest schedule. But perhaps the pollsters are almost right. Texas probably has the best returning talent and should end ahead of OU or Bama. But I would think Bama ends the season ahead of OU since Bama has an easier schedule.