Are Three Pours Required at Baptism?

For the purpose of validity, it does not matter if the water is poured once, twice, or thrice. All that is required is the pouring of water and the trinitarian formula.

The Catechism of Trent, The Sacrament of Baptism:

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.