At 3 weeks old, I would suggest to continue keeping the feeders full for them. Someone with more experience may weigh in differently, but for babies I think they need to have it available to insure they get the nutrition required.
When chickens get older, some people do feed them twice a day, rather than leaving it out all day for them. The concern is that the chickens lower in the pecking order may not get as much food as they need.
I'm not sure I'd worry about obesity at such a young age as much as I would worry about their nutritional needs.
Mine stay outside in their run all day and forage around, rarely going to their food dish in the coop at all, even though it's full of food and available 24/7. See how yours act when they get older.
My 1 cent,
Penny
