Roman Catechism 16.34

Chapter 16 — Part II: The Sacraments — Baptism

Section 34. It is a matter of indifference whether the ablution be performed once or thrice. For it is evident from the Epistle of St. Gregory the Great to Leander that Baptism was formerly and may still be validly administered in the Church in either way. The faithful, however, should follow the practice of the particular Church to which they belong.