Three

Three is the number of the ideal or ultimate.

The holy of holies (in God's temple) was a cube (three dimensional square), 20 cubits on a side (1 Kings 6:20). Some say the huge cube in Revelation 21:16 is a reference to this cube (others say it is not actually a cube).

The ultimate man is given by the number 666 (six is the number of man, coming up!).

It is believable, then, that God should be three persons in one (although this is not sufficient). We can know that God is multiple persons; because God is love (1 John 4:8), and love is a relationship. The proof is related to the number of witness (2):

"For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." - 1 John 5:7

Any two members of the Trinity can bear witness to the third. I mentioned the baptism of Jesus. In addition, we come to know God (the Father) through the preaching (Romans 10:14) of the Word (Jesus), and the indwelling of the Holy Spirit (John 15:26-27). This passage in John also describes (among others) how Jesus and the Father send us the Comforter (the Holy Spirit).