At only 3 weeks I would be thinking either tummy pains, or reflux or something like that. There are natropathic medicines you can get for wind, pains etc. Brauers have a good selection. If you go to your chemist they can tell you what is safe. go for the ones with all natural ingredients. My baby grunted for the first 9 weeks (24 hours a day) and it drove me insane. The colic medicines I used were good, but she really just had to grow out of it.
At 9weeks, we then let her settle herself which was fantastic!!
good luck xo