Is it bad that i've been coughing up phlegm for about 3 weeks now?

No: Most likely these symptoms will pass. Allergies have been really bad this year due to the lack of rain.

Needs Eval: A persistent cough that is productive needs to be evaluated. You may have chronic bronchitis, a form of copd. Other possibilities include bronchospasm of any etiology, gerd, post-nasal drip or even a combination of factors. It needs evaluation so proper treatment can be rendered. If you smoke, you need to quit today!