How did Paul receive three years of training from Jesus if he was not with the other apostles in Palestine?
Paul did not need to be in Palestine with the Twelve. He received a private tutorage from the Lord Jesus Himself, spending three years in Arabia, likely at Mount Sinai, so that in time spent with the Lord he was not one whit behind the twelve.
Les explains that Paul's opponents contrasted him with Peter and the others, who had three years with Jesus walking up and down the dusty roads of Palestine. Paul did not spend those years in Palestine, but Les suggests he had a private tutorage from the Lord Jesus Himself for three years, likely in Arabia at Mount Sinai. That gave him the same full authority of time spent with the Lord, so he was not one whit behind the twelve. Paul also argues in 2 Corinthians 11:22-23 that he is a Hebrew, Israelite, and seed of Abraham, and that his labors, stripes, prisons, and deaths exceeded theirs.