Can a virus last longer than 3 weeks? bloods normal. doctor insists virus but doesn't know what kind? fatigue, muscle aches, fever, nausea subsided

Yes it is possible: Usually a viral infection stay in the body around a week. But plenty of viruses stay longer than that. They may stay upto two weeks and in some circumstances some viruses like hepatitis B or C can be a life long infection. Sometimes its the weakness that persists longer than viruses themselves. If you are feeling otherwise better, then give it a week more and then see your doctor again. Good luck