Passed kidney stone about 3 weeks ago and still having mild discomfort in side. how long can this go on? or should i see a doctor?

Definitely....: especially if you haven't been working with a doctor already. Usually discomfort associated with passage of a kidney stone will have dissipated by 3 weeks. Questions needing answers are 1) Is there an associated infection?, 2) Are there more stones?, 3) What is the composition of the stone? (it should be/have been analyzed). See a kidney specialist or urologist to review. Good wishes:)

Need a reevaluation: I'm concerned you may have an additional stone that hasn't passed yet as you shouldn't be continuing to have pain 3 weeks after passing a stone. You should see your physician for evaluation and a referral to a urologist if not already seeing one. For more information on kidney/ureteral stones see: http://bit.ly/2aBx4ww and http://bit.ly/2bjmabS