Complex sentence: Goku trains under Kami for the next three years, once again reuniting with his friends at the "Tenkaichi Budōkai", where he wins against Piccolo; whose life he spares as it would also kill Kami.
Split into simpler sentences: Goku trains under Kami for the next three years, once again reuniting with his friends at the "Tenkaichi Budōkai". There, he defeats Piccolo, whose life he spares as it would also kill Kami.