When Jesus appeared to St. Faustina during the 1930s in a private revelation, he placed a great emphasis on a particular three o'clock prayer in honor of his Divine Mercy.
St. Faustina wrote down his words in her Diary.
Jesus gave to St. Faustina the Divine Mercy Chaplet, but also a particular prayer that can be said when the clock strikes three -- the traditional hour of the crucifixion.
It is a beautiful prayer and one that can be easily memorized to be prayed at that hour each day.