...didn't see your other post, but from the available info in this post, this snake needs to be assist-fed and SOON. Make sure you have a spot in the cage that is low 90's, good humidity, and a small hide. Assuming your environment is good, this sounds like a small specimen of whatever specie it is (some might say a 'runt'), and it may have been weak from the start. Perhaps it might hit on a fuzzy as opposed to a pink. But this has to happen SOON, like in the next 2 days. Otherwise, an assist-fed large pink or small fuzzy is what this snake needs, or a prepared food (ie Zu-preen) tube-fed. Bottom-line, lack of a tongue-flick is not a good sign. This snake is weakening rapidly, for 3 weeks is TOO LONG for a baby to go without nutrition. If you are unfamiliar with assist-feeding have another experienced herper do it for you. Good luck!
Scott J. Michaels DVM
P.S. Wait about 4 days before giving a second meal, no larger than the first. Very GRADUALLY increase the size of its meals.