What do 3 week old baby mice eat?

I found excellent advice and instructional videos on YouTube. Try using a clean small paint brush style brush, about the size of an eyeliner brush. For food, I used watered down skim milk until I was able to get kitten formula. The baby mouse will suck the liquid off the end of the brush - it works very well. Then very gently rub it's tummy after feeding to stimulate digestion. Good luck.

Baby mice are breast fed by their mother until they are about 3 weks old when they can eat normal mouse food.

If it doesn't have it's eyes open yet it still drinks it's mothers milk but if it's eyes are open she starts weining it to eat mouse food....

Their mother's milk