Adam Clarke on Galatians 1:18

After three years I went up to Jerusalem to see Peter - These three years may be reckoned either from the departure of Paul from Jerusalem, or from his return from Arabia to Damascus.

To see Peter - Ιστορησαι Πετρον, to become personally acquainted with Peter; for this is the proper import of the verb ἱστορειν, from which we have the word ἱστορια, history, which signifies a relation of things from personal knowledge and actual acquaintance. How far this is, now, from the sense in which we must take the word, ninety-nine of every hundred of our histories sufficiently show. They are any thing but true relations of facts and persons.

And abode with him fifteen days - It was not, therefore, to get religious knowledge from him that he paid him this visit. He knew as much of the Jewish religion as Peter did, if not more; and as to the Gospel, he received that from the same source, and had preached it three years before this.