God is most certainly One, but He is triune in nature. If you want an example of one thing having three forms look no farther than water. It can be a liquid, a solid when frozen, and a gas when vaporized.

Also, the scriptures prove that God is triune in nature.

Jesus gives the same teaching three different times in the gospels, but there is something different about each teaching that expressed God's triune nature:

Matthew 10:16-19
16 "Behold, I send you out as sheep in the midst of wolves; so be shrewd as serpents and innocent as doves.
17 "But beware of men, for they will hand you over to the courts and scourge you in their synagogues;
18 and you will even be brought before governors and kings for My sake, as a testimony to them and to the Gentiles.
19 "But when they hand you over, do not worry about how or what you are to say; for it will be given you in that hour what you are to say. 20 "For it is not you who speak, but it is the Spirit of your Father who speaks in you.

Mark 13:9-11
9 "But be on your guard; for they will deliver you to the courts, and you will be flogged in the synagogues, and you will stand before governors and kings for My sake, as a testimony to them.
10 "The gospel must first be preached to all the nations.
11 "When they arrest you and hand you over, do not worry beforehand about what you are to say, but say whatever is given you in that hour; for it is not you who speak, but it is the Holy Spirit.

Luke 21:12-15
12 "But before all these things, they will lay their hands on you and will persecute you, delivering you to the synagogues and prisons, bringing you before kings and governors for My name's sake.
13 "It will lead to an opportunity for your testimony.
14 "So make up your minds not to prepare beforehand to defend yourselves;
15 for I will give you utterance and wisdom which none of your opponents will be able to resist or refute.