i have had a uti infection for three weeks now. none of the pills are working

If your symptoms are getting worse you do need to contact your physician. You may need stronger antibiotics and get them I.V. With some infections, if you have taken certain antibiotics for long periods of time, can become immune to those antibiotics. I know that now with kidney infections they wont treat just because you have the germ, they wait until you get sick. If they treat because a germ shows up in your urinalysis then there will not be an antibiotic to treat when you get sick from the germ. This may hold true to UTI's also. I would contact the physician for sure and you may need to go into the hospital for an I.V antibiotic.