The problem with many people when they contemplate about the Trinity is that they try to view the Trinity from the perspective of Polytheism. You cannot find the word 'Trinity' in the Bible nor any notion of three Gods. The Bible clearly says that God is one, not three.
Hear, O Israel: The Lord our God, the Lord is one. Love the Lord your
God with all your heart and with all your soul and with all your
strength. (Deuteronomy 6:4-5, NIV)
One of the teachers of the law came and heard them debating. Noticing
that Jesus had given them a good answer, he asked him, “Of all the
commandments, which is the most important?” “The most important one,”
answered Jesus, “is this: ‘Hear, O Israel: The Lord our God, the Lord
is one. Love the Lord your God with all your heart and with all your
soul and with all your mind and with all your strength.’ (Mark
one Lord, one faith, one baptism; one God and Father of all, who is
over all and through all and in all. (Ephesians 4:5-6, NIV)
You believe that there is one God. Good! Even the demons believe
that—and shudder (James 2:19, NIV)
The Trinity defines God, His personality, His nature and His attributes. We don't have three Gods. Some people are confused with this idea that they think that Christians have three Gods. If we try to relate the Trinity with polytheism, it is a valid question to ask why there are only three incarnations of God and why not four or more. But Christianity is monotheism, believing in one God, a God who never comes through many forms with different natures. God came as He is, without changing His nature.
All complex substances are composed of many parts. For instance, we may divide man into three parts - body, soul and spirit. We may divide an egg into three parts - shell, white and yolk. God has three parts - Father, Son(Word) and Spirit. We may understand this as - God, God's Word and God's Spirit. Parts of a man can be separated. For instance, when we die, our body remains on earth while our spirit is either in Hell or Heaven. The unique quality of God is that, all three parts of God can function like a person. The Father has the function of the head and controller of all things, the Son(Word) has the function of a mediator between God and men, the Holy Spirit has the function of transforming and sustaining men. All three parts have their own specific roles and yet they act in one mind. They can never go against each other because they are one. This is our God, who is awesome, full of love and full of grace.
Trinity is the nature of God and God never changes. Trinity is what God is and it will never change. Our God will always be a Triune God, in the past, now and forever more.
“I am the Alpha and the Omega,” says the Lord God, “who is, and who
was, and who is to come, the Almighty.” (Revelation 1:8, NIV)
Jesus Christ is the same yesterday and today and forever. (Hebrews