This is something that many have problems with. Because we claim to serve ONE GOD and still there is God the Father, Jesus the Son of God, and the Holy Spirit. Well is it three or one?
These are three, but you can’t have one without the other so they are ONE GOD.
Let me try to make it clear with some materials most of us have or have had.
Think about a laptop. It is ONE COMPUTER. Still it has a KEYBOARD, a SCREEN, and a PROCESSOR. You would not think of buying one part, because you need all three parts to have ONE COMPUTER.
Or think about a RADIO-CASSETTERECORDER (do they still exist?). It is ONE UNIT. Still in that one unit there are a RADIO, a CASSETTE RECORDER, and a SPEAKER SET. In India I once heard someone refer to it as a “three-in-one”
The term THREE-IN-ONE will also sound familiar when you think of a PRINTER in combination with a SCANNER and a COPIER.
So in daily life we know of things THAT ARE THREE but IN FACT THEY ARE ONE. And when you buy them you buy them AS ONE. Nobody finds this strange.
Now back to God.
He is ONE GOD and in this one God there is GOD THE FATHER, JESUS Christ, and the HOLY SPIRIT. THREE IN ONE and you can’t have one without the other.
Although I know the examples I used are not 100%. And this is a very weak way to clarify the greatness of the trinity of God, I still hope it helps you a bit to understand the term “TRINITY”, the UNITY OF THREE which is ONE.