When I had a tiny kitten found on the road that was about 3 weeks old my vet had me put her on a pediatric kitten food.
I also gave her kitty milk, my vet seemed to think she didn't need it...but thats a whole different story..love my vet...but our tiny Diana only lived to be approximately 9 weeks old because of a genetic disorder..I think someone dumped her because they knew she had serious issues.
This was her around 9 weeks
I think the kitteh you posted is more than 3 weeks. It should be seen bye a vet to make sure it is fine, be de-wormed..etc and maybe talk to the vet about food..but by the looks of it kitteh looks like it will be ok without mamma too me.