Brill that takes time pressure off you. In terms of diet, as 3 weeks old they should be fairly keen to try lots of different things, ideally they'd still be feeding from mum too but as its not an option you will ahve to do your best. They need high protien foods that are also rich in vitamins and minerals. Also at this age you often set there approach to food for life too. I tend ot offer my kittens a wide range of soft foods that are high in protien about once or twice a day (unless there fat then they get it less often). This varies from day to day (as feeding variety makes them less suspicious of new foods in the future) but heres some ideas;
tinned cat / kitten food - aim for natural ranges so its not full of rubbish fillers
egg - scrambled, hard boiled, microwaved in some cooked white rice, or soaked into bread then microwaved, they often take it better with the carbs
Egg food - sold for rearing birds such as budgies and parrots, this is really handy to have in as its a dry food you soak in warm water or even better puppy/kitten formula
fish/seafood - stuff like mackrel or sardines in tomatoe sauce, cooked fish (even better on the bone) cooked shellfish and so on. Mix with some simple carbs like couse couse or rice if they are reluctant to try it at first.
Chicken, cooked and ideally on the bone.
Baby foods - these can be useful as they are fairly high protien, especially some of the porridge mixes, however i dont think they should be used as a sole source as they arent potent enough for young growing rats (human babies have a different growth rate to rat babies)
As well as this i feed a high protien rat mix, in my case its one i mix up myself specifically for babies and mum, however you can either use a higher protien pellet (i think oxbow has a young rat mix) or use a staple rat mix and add one scoop of a high quality dog food (protien content around 25%) to 9 of the staple rat mix. At this age they are perfectly able to start eating dry food but need the support of lots of wet food too. Assuming the babies are doing well but not too fat i would give them as much as they will eat of this.
Extras - theres some really useful extras that baby rats should get to help meet there extra needs for vitamins etc
Puppy / kitten formula - over here theres a brand called lactol which is used heavily for rat kittens, soya baby formula is also an option. Feed this in a small water bowl alongisde one with water in or in a shallow dish.
Bones (cooked) these are a great source of calcium and also if they have marrow in a great source for other minerals too. I tend to give my babies a lot of cooked chicken bones as its what i have in, or sometimes i will buy them a big roasted bone sold for dogs.
Veg - giving them a variety (start with a small amount and work up) really helps, but the key veg type at this age is dark green leafy stuff, like kale, cabbage, spring/fresh greens, packchoi and dandilion
Salmon oil - if you can find this its brill for babies, packed full fo vitamin d, give a few drops on a wet meal once a week.
Seaweed powder (or torn up nori sheets sold for sushi making), another great source of vits and minerals, i add a pinch to a wet meal once a week.
After you;ve got the diet sorted the next focus is on socialising them and getting them used to you and a variety of noises and things that go on in your home. Try and get them out for at least 30 mins a day. At this age they do tend to collapse after not too long so i often get them out more than once in a day. They are old enough now that they will benefit from interacting with different objects. I tend to set up a table top with lots of bits and bobs, say toilet roll tubes, cardboard boxes, kids toys etc. The idea is to give them lots of new things and change it regularly so they learn not to be scared of new things. If you can do the same with different people too its brill as well, i tend to invite quite a few friends round to help socialise the babies over the period of 3-6 weeks, even just being gentley handled by a new smelling personm can make a difference.