OK...3 week old babies still have the roundish face, and they are able to eat solid foods without mom. Lets get an idea of the age of your baby first. Using my orphan boy that I adopted at 3 weeks of age.
Here's Dilbert at 3 weeks old (he's holding a rice crispie)
Here he is at 3.5 weeks
Here he is at 4 weeks (when they are usually being weaned)
If you think your boy is younger, here's some of the Bronlings for reference
Here is Megan and Angel at 2.5 weeks old
Here is Bear at 17 days old...his eyes just opened 3-4 days before...note the sprawly body and squinty eyes.
If your baby is 3 weeks or thereabouts, I would invest in human baby formula (powdered), baby cereal, I have used strawberry Ensure or Boost myself and that is what I fed little Dilbert.
I had a tiny bowl of water (it was a jewellery ceramic thing) and I had some Suebee's mix and broken up lab blocks in there as well. I gave him warmed frozen peas, and corn which he loved. I wouldn't give him yogies just yet since its only sugar and has no nutritive value for a little baby.
At 4 weeks, I had a tiny waterbottle here that I put up in his cage and he figured it out pretty quick.
I kept Dilbert on the baby cereal with formula for awhile to make sure he got all the extra nutrition a growing boy needed.