Avoid a lot of greens, especially iceburg lettuce, as this can give young rats the runs. Things like dry cereals, soft nuts, etc. are great for chewing. You can break up some lab blocks too. Also softer foods like boiled eggs, canned tuna, bits of chicken are great for younger rats and very nutritious. At 3 weeks your rat can be weaned. It's a bit early but the rat should have erupted teeth and be able to chew. My rats that age, though still nursing, are using their mother's water bottle and digging into the dry food bowl and kitchen scraps just fine.

Now where are the pics!!! LOL