Romans 3:9-20

In verse 9, Paul asks, “[A]re we better than they?” The pronoun “we” includes all who are the descendants of Abraham, Isaac and Jacob. The “they” are Gentile believers. The church at Rome was made up of both Jew and Gentile believers. Paul had reason to believe that there were Jewish believers in the church who might think themselves to have favor with God because Jews were given the scriptures and were of the lineage of Abraham.
In answering his own question, the apostle states that all men, whether Jew or Gentile, are under sin.