Why did Paul spend three years alone before preaching?

Paul needed those three years to shed the demands and legalism of Judaism that had weighed him down and to absorb the revelations he would later bring out, especially in Ephesians, Philippians, Colossians, and Philemon. This prepared him to write with fresh understanding.

Les explains that Paul was coming out from under Judaism and its demands as a religious leader, now set free. He had three years to shed himself of all that and to absorb the truths he later brings out in his prison epistles—Ephesians, Philippians, Colossians, and Philemon. Les suggests these revelations were probably given to him while imprisoned in Caesarea before Rome, so that by the time he reached Rome they were fresh on his mind as he wrote. This period separated these doctrines from legalism and Judaism, yet they still fit with all Scripture.

What was Paul doing during the three years alone before he preached?

During that three-year period, Paul was shedding Judaism and its religious demands and absorbing the doctrines he later set forth in his prison epistles. He was being set free from legalism and receiving revelations, which were fresh when he later wrote from Rome.