If you wish to file a complaint under Title III regarding a private business or public accommodation, you may file a complaint at https://ada.gov/complaint/ (Federal Civil Rights Division of the Department of Justice) or https://dfeh.ca.gov/business-e... (California Department of Fair Employment and Housing (DFEH)).
If you have an issue related to the Fair Housing Act, you may file a complaint with the Federal Office of Housing and Urban Development Fair Housing and Equal Opportunity (FHEO): https://www.hud.gov/program_offices/fair_housing_equal_opp/online-complaint.