Is Paul in Romans 3 speaking about Old Testament saints as well as New Testament believers?
Yes. Paul’s argument in Romans 3 includes both the New Testament believer and the Old Testament saint. Their sins were totally atoned for by the shed blood of Christ, so the same righteousness and justification apply. Paul is not limiting the passage to New Testament believers alone.
Les explains that Paul is reminding us that he is not only talking about the New Testament believer; he is also bringing in the Old Testament saint. The Old Testament saint’s sins were now totally atoned for by virtue of the shed blood of Christ. Les also notes that once the true atonement was accomplished, there was no reason to send people down into paradise, and paradise was taken directly to heaven. Paul constantly hammers home that we have nothing good, no righteousness, and nothing to barter; we are helpless and hopeless, but that is when God steps in, so He might be just.