The Three Pure Ones as One was no empty saying.
In Lifetime, before the Creation of Heaven and Earth, the Great Dao became the Three Pure Ones.
Each of them embodied one aspect of the Dao.
Beginning, flourishing, ending.
Only when the Three Pure Ones affirmed one another could they become the Dao Fruit, a complete Dao.
So even if each of the Three Pure Ones did everything possible through later cultivation to master the Daos represented by the other two, achieving flawless perfection, they still could not attain the Dao alone because of their influence on one another.
The Three Pure Ones had not seen this clearly at first. Only after fighting their way into the Heavenly Court and seeing the Jianmu Fruit did they recognize the truth.
When Primordial Heavenly Venerable had sighed, "The matter of the Dao Fruit admits no reliance on external things," one reason was that he had seen the Jianmu Fruit itself could not bring a Dao Fruit Embryo to true transcendence.
The other was that he had recognized the truth of the Three Pure Ones as One.
If they wanted to transcend, they could never succeed by carrying on as before—treating one another as enemies, fighting each other, dragging each other down and stabbing each other in the back.
That was why Primordial Heavenly Venerable and Lingbao Heavenly Venerable had secretly fought to decide who would take the lead in their transcendence, and to settle their many old grievances.
If nothing else, the Investiture of the Gods War alone had left Primordial Heavenly Venerable and Lingbao Heavenly Venerable with Great Karma between them.
If they were to vie for the Dao Fruit, all of it had to be settled. From then on, they would truly be the Three Pure Ones as One.
But