Did any actor appear in all 3 Law and Order TV shows?

Yes, actor S. Epatha Merkerson appeared in all three "Law & Order" series: the original "Law & Order," "Law & Order: Special Victims Unit," and "Law & Order: Criminal Intent." She is best known for her role as Lieutenant Anita Van Buren in the original series, which she portrayed for 16 seasons. Her appearances in the other series were guest roles, further solidifying her connection to the franchise.