Saint John Eudes, whose feast day we celebrated last Monday, once said: "Never separate what God has so perfectly united.So closely are Jesus and Mary bound up with each other that whoever beholds Jesus sees Mary; whoever loves Jesus, loves Mary; whoever has devotion to Jesus, has devotion to Mary."
The Three Hearts devotion is also easy to understand- it is nothing other than honoring the Three Hearts of the Holy Family of Jesus, Mary, and Joseph.