1. A flat and neat refusal
If roommates asked you to help bring food at first, you refused directly and let them buy their own food. If you always bring food for them at first, they will take it for granted that you bring food for them. Then if you don't want to bring food for them later, they will be unhappy and may speak ill of you behind your back, so you should simply and neatly refuse their request to let you bring food for them and try to reduce the time in the dormitory.
Polite refusal
You're going to eat, and your roommate wants you to bring your meal. You can ask her what she wants to eat first. If she wants to eat from the first canteen, then you tell her that what you want to eat is in the third canteen, not on the road. At this time, you are usually embarrassed to bother to make a detour for her, or you invite her to the canteen and tell her that the food cooked now is better than the packaged one. When you are ready to go back to the dormitory outside, your roommate asks you to help bring food. You can reply later.
Learn to say no.