The U.S. is the only democracy in the world that deprives the residents of its capital city of the basic rights granted to other citizens. Even more damaging than the lack of congressional representation is the colonial-style control that Congress exerts over the district. Adding one, or three, D.C. representatives to the 535 members of Congress would, by itself, do little to solve this problem. Unaccountable power is by its nature abusive. The places where unaccountable power is exercised are, and must be, dysfunctional.
Unaccountable power is uninformed.
Members of Congress don't know this city. They don't know what's right for its people. They approve the budget and all the legislation, but they do not themselves have to live with their decisions. They foist pet projects on citizens who are perfectly capable of deciding these issues locally. They prevent the district from taxing income where it is earned.