Juseige comes from the Larger Sutra of Immeasurable Life.
In that sutra the Bodhisattva Dhamakara, who becomes Amida
declares to the Buddha Lokesvararaja his intentions
to become a Buddha himself.
The Three Sacred Vows, refer to Dharmakara's pledge to:
1. Establish the most incomparable Vow in the world
2. Become a great provider and save the poor and suffering
3. Attain Buddhahood and have his Name heard throughout
the ten quarters of the universe.