The mystagogy of the celebration
The essential rite of the sacrament follows: Baptism properly speaking. It signifies and actually brings about death to sin and entry into the life of the Most Holy Trinity through configuration to the Paschal mystery of Christ. Baptism is performed in the most expressive way by triple immersion in the baptismal water. However, from ancient times it has also been able to be conferred by pouring the water three times over the candidate's head.
Source: Catechism of the Catholic Church (Vatican.va)