I had green phlegm for about 3 weeks with a cough and phlegm is no longer green is it necessary to still seek medical attention?

Maybe not. If u r feeling better and less sick, no temperature u may have had a viral infection and ur body has won. If u continue 2 improve u may b ok. But any worsening of ur cough or u have a color change again b cn. That would b what i personally would do. 4 peace of mind u should c u r pcp?