John Calvin on John 21:14

14. The third time. The number three refers to the distance of time. Christ had already appeared to his disciples more than seven times, but all that had been transacted in one day is included in one manifestation. The Evangelist, therefore, means that Christ had been seen by the disciples at intervals, in order to confirm their belief of his resurrection.

John 21:15-19

15. When, therefore, they had dined, Jesus saith to Simon Peter, Simon (son) of John, lovest thou me more than these? He saith to him, Yea, Lord, thou knowest that I love thee. He saith to him, Feed my lambs. 16. He saith to him again the second time, Simon (son) of John, lovest thou me? He saith to him, Yea, Lord, thou knowest that I love thee. He saith to him, Feed my sheep. 17. He saith to him the third time, Simon (son) of John, “Simon (ills) de Jona;” — “Simon (son) of Jonas.” lovest thou me? Peter was grieved because he said to him the third time, Lovest thou me? And he said to him, Lord, thou knowest all things: thou knowest that I love thee. Jesus saith to him, Feed my sheep. 18. Verily, verily, I tell thee, When thou wast young, thou girdedst thyself, and walkedst whither thou wouldest; but when thou shalt be old, thou shalt stretch out thy hands, and another will gird thee, and will carry thee whither thou wouldest not. 19. And this he said, signifying by what death he should glorify God; and when he had spoken this, he saith to him, Follow me.