I want to know if it is necessary to say "I baptize you in the name of the Father, and the Son and the Holy Spirit" during a baptism? And is immersion valid if it is done once in the name of the Father, once in the name of the Son and once in the name of the Holy Spirit (i.e dipping one thrice)?
I know you're being bombarded with questions. I pray God keeps you around for a very long time to come.
The phrase "in the name of ..." means "by the authority given by ..." in more modern English. However, we still sometimes say, "Stop in the name of the law!" There is no set formula of words that are required to be spoken at a baptism. See: What is the proper thing to say as you are baptizing someone? Whether you say it is by the authority of the Father, the Son, and the Holy Spirit or you do it because of the authority grants to you, it doesn't change what is being done.
Baptisms, where a person is immersed three times, are practiced by the Orthodox Church and the Church of the Brethren. The idea is drawn from Matthew 28:20; however, the passage discusses one baptism done by the authority of the three beings of the Godhead. It also misses the symbolism of baptism as discussed by Paul in Romans 6:3-7. Baptism is a representation of the death, burial, and resurrection of Jesus. Going down into the water represents Jesus' death, being underwater represents his burial, and coming up from the water represents his resurrection. Three immersion doesn't carry the same representation. Then there is the problem that in the many mentions of baptism in the New Testament, not one indicates the person being baptized was immersed three times.