Had Israel already been forgiven for killing their Messiah by the time of Acts 3?
No. By Acts 3, the nation of Israel had not yet been forgiven for rejecting, killing, and crucifying their Messiah. Peter, addressing men of Israel, charged them with denying the Holy One and killing the Prince of Life, then called them to repent, showing that this guilt remained on God's mind.
Les explains that in Acts 3 Peter directly charges the men of Israel with delivering up, denying, and killing the Prince of Life. Since verse 19 begins with "Repent," Les teaches that the nation had not yet been forgiven for rejecting and crucifying their Messiah; this was still on God's mind. He also cautions that this must never be used as an excuse for anti-Semitism, noting that many religions over the last 1900 years have wrongly accused the Jews as God killers.