Hi.. Can understand your concern.. As per your complain, in upper wisdom tooth extraction there are frequent chances that the sinus gets opened up but the healing takes place as the blood clot is formed and as the healing progresses the sinus also heals. If the sinus would be still open there must be some air or fluids passing between nose and mouth and opened sinus for three weeks must have got infected with some symptoms like pus discharge, pain and pressure on face in sinus area etc but it does not seems evident in your case. The other thing is that redness of mouth can be if you are using very hot water to gargle and if you do for three times a day with hot water the redness and inflammation can happen.. What is advised is to use luke warm water and not hot water. As per your question it seems you don not have any pain in the extraction area and low grade fever after three weeks can be as a result of viral infection and not related to extraction. Still if you feel unconvinced you can consult an oral surgeon to get a clinical evaluation done so that he can see clinically that what is the status of healing and whether there is any infection in the region causing fever or it is not related to it. For redness of mouth you can take cold fluids which can help you get relieved and avoid hot and spicy foods to avoid any irritation of mouth. Hoping you get well soon.. Thanks and regards.. Dr.Honey Nandwani Arora.