What is required of a young man who left Islam for three years, and did not fast the month of Ramadan for one year during that period, then returned to Islam?

All praise is due to Allah, who guided you to His religion. As for making up the fasts you missed during your apostasy, this is a matter of scholarly disagreement. The Shafi'is and Hanbalis made it obligatory, while the Hanafis and Malikis did not. The cautious approach is to make them up, in order to clear oneself of obligation and to perfect one's repentance.