That's pretty common. You can use a soft cloth or baby toothbrush to brush their gums and sort of swipe their tongue. Btw, the earlier you start the tooth-brushing, the easier time you will have. Do it before they have teeth and they accept it so much more easily.
My LO is also 3 wks old and I've noticed this, but didn't think it was anything I should be concerned about. Is this something I should be removing from the build up on her tongue?
January 2014:BFP!!!! Ectopic pregnancy ruptured at 6wks1day. Left Fallopian tube removed. Noted during surgery the right Fallopian tube is severely damaged from 2004 ectopic pregnancy.
April 8 2014: IVF#1 w/ISCI: 10 received, 5 mature, 3 fertilized. Day 3 transfer of all three embies.
April 22, 2014: BFP! beta #1: 80 beta #2: 211 One nugget!
I thought that's what my DD had when she was an infant, but it ended up being thrush, which I learned is very common. Not saying that's what your child has, but I didn't even know it existed when mine had it so just a heads up.
Re: Formula Build Up on Tongue