I've had thick and runny and yellow drainage for 3 1/2 weeks, tried everything and it won't go away it's even hard to swallow is it a sinus infection?

Yes: A bacterial sinus infection usually takes 7-10 days to develop. Now that it has been 3.5 weeks, this sounds most likely to be an infection. You should see a doctor to be placed on an antibiotic for 7-10 days. It would be advised that you use nasal saline spray 2-3 times a day purchased over the counter and i like to recommend Mucinex (guaifenesin) taken twice daily for about a week to help thin the mucus.