Some denominations only do immersion once.
From the Catechism of the Catholic church: paragraph 1239 "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.
1240 In the Latin Church this triple infusion is accompanied by the minister's words: "N., I baptize you in the name of the Father, and of the Son, and of the Holy Spirit." In the Eastern liturgies the catechumen turns toward the East and the priest says: "The servant of God, N., is baptized in the name of the Father, and of the Son, and of the Holy Spirit." At the invocation of each person of the Most Holy Trinity, the priest immerses the candidate in the water and raises him up again"
