Paul has spent the best part of chapters 1 and 2 building up a case of guilt, of sin against, first the gentiles and then the Jews. So that he can get to the key verse of chapter 3. Verse 9 says all are under sin.
The weight of the realisation that this includes and describes you and me is weighty and hard. But Paul goes on to say that we are totally sinful – every part of us is affected. We especially sin as we fail to honour and love God.
What might seem hopeless and bleak, is just that…except for Jesus. There we find Grace!