John Calvin Commentary Galatians 1:18

"Then after three years I went up to Jerusalem to visit Cephas, and tarried with him fifteen days." — Galatians 1:18 (ASV)

Then after three years. It was not until three years after he had begun to carry out his apostolic duties, that he went up to Jerusalem. Thus, he did not, initially, receive his calling from men. But to prevent any thought that he had separate interests from theirs, and wanted to avoid their society, he tells us that he went up for the express purpose to see Peter. Although he had not waited for their approval before taking up the office, it was not against their will, but with their full consent and approval, that he held the rank of an apostle. He wishes to show that at no time was he in disagreement with the apostles, and that even now he is in full harmony with all their views. By mentioning the short time that he remained there, he shows that he had come, not to learn, but solely for fellowship.