In the New Testament:
Jesus' ministry lasted three years, covering three Passovers [John 2:13; 6:4; 11:55-12:1].
Mary stayed with Elizabeth about three months [Luke 1:56]
Jesus was missing for three days when He was 12 years old [Luke 2:46]
Jesus took Peter, James, and John up on the Mt. of Transfiguration [Matthew 17]
Jesus arose from the dead on the third day.[Genesis 1:9-13]
Saul was blinded for three days [Acts 9:9]
The theological virtues are faith, hope and charity [1 Corinthians 13:13]
The heavenly Jerusalem has three gates on each of its four sides [Revelation 21:13]
Christians saw three as symbolic of the Trinity, the triune nature of God: Father, Son, and Holy Spirit.
Three is also recognized as the number of the Holy Spirit.
There are three divisions of time: past, present, and future
There three persons in grammar
Thought, word, and deed complete the sum of human capacity.