I have had a sore throat for over 3 weeks what is wrong?

Long time..... Best to see your otolaryngologist for examination and appropriate treatment.
Sinus or reflux. If your symptoms started with a cold... It could be a sinus infection. If it is more irritation than pain... Acid reflux could cause it. Should also consider allergy depending on where you live. Chronic tonsillitis is another consideration. I would see an ENT in your area.