80-85 degrees F should be plenty warm for a 3 week old, unless it seems to be standing under the heat lamp. I would treat for possible coccidiosis with Corid (amprollium.) A feed store should sell it for chickens and cattle. If you have a vet they could give it to you, or might prescribe a sulfa antibiotic instead. Symptoms of coccidiosis are lethargy, not eating, being hunched or puffed up, and runny poops (sometimes with blood.) Occasionally a chick can be a failure to thrive chick due to some internal problem.