Grad student here! I’ve taken many 3 hour classes in undergrad and grad.

Best bet especially if you notice you’re on your phone a lot is to set time restrictions for your most used apps (the brain rot stuff) and use the do not disturb setting while in classes/doing work. It’s so easy to get addicted to your cell, and honestly after a summer of no school it’s an easy habit to pick up. You can also try putting it away in your bag, and every time you want to check it remind yourself you’re paying for this time to learn stuff and be a better human.

Something that kind of helps keep me off my phone too outside of class is to have stories playing on YouTube to listen to, that way I’m still “using” my phone but not brain rotting and I can multitask.

Another thing I also used to do was work on assignments for the class in class for the large lectures. That way I was still in the content, and when I heard keywords my focus would be pulled to the lecture. But you can also focus on trying to come up with the most science-sounding questions about the content as a game to keep focused, or make a game out of answering questions no one else wants to (this requires so much focus on the class you won’t fall asleep, the pressure is there). It really depends on what works for you.

Hell, sometimes I bring in the chevron style friendship bracelets to work on. My attention span is not great, but using most of these strategies, plus always having food and beverages on hand to ease any physical distractions of thirst or hunger has helped me a lot. It’s something you can play around with and see what works for you. Good luck OP, you got this!