Why was the crowd gathered for Peter's sermon in Acts 3?

The crowd gathered because of the miracle that happened to the lame man as a result of Peter and John. The Jews came together in amazement at his healing, and Peter used that opportunity to preach to them about the God of Abraham, Isaac, and Jacob.

Les explains that in Acts 3, Peter is again preaching to a massive crowd of Jews. This crowd had gathered because of the miracle that happened to the lame man through Peter and John. When Peter saw the amazement of all these Jews, he addressed them as “Ye men of Israel,” with no mention of Gentiles. He asked why they marveled or looked earnestly on them, as though by their own power or holiness they had made the man walk. Peter attributed the miracle to the God of Abraham, Isaac, and Jacob, who glorified His son Jesus. He reminded them that they delivered Jesus up and killed the Prince of life, whom God raised from the dead. So the gathering was prompted by the healing, not by a planned sermon.

Acts 3:1

Now Peter and John went up together into the temple at the hour of prayer, being the ninth hour.

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Why did the Jewish crowd gather to hear Peter preach in Acts 3?

The Jewish crowd gathered in Acts 3 because of the miracle that had happened to the lame man through Peter and John. Their amazement at his healing drew them together to hear Peter preach, and Peter then addressed them as men of Israel, pointing to the God of Abraham, Isaac, and Jacob.