Mark 1:9-11 (CSB) He proclaimed, “One who is more powerful than I am is coming after me. I am not worthy to stoop down and untie the strap of his sandals. I baptize you with water, but he will baptize you with the Holy Spirit.”
In those days Jesus came from Nazareth in Galilee and was baptized in the Jordan by John. As soon as he came up out of the water, he saw the heavens being torn open and the Spirit descending on him like a dove. And a voice came from heaven: “You are my beloved Son; with you I am well-pleased.”
If there are three people and just one God, how can all three persons be one God?
Three persons in this room does not have one will. Triune God does have one will.
The best way I explained the Triune God to my kids is explaining how my wife and I try to have one will but we do so imperfectly. That is why my wife and I have to talk all the time especially when it comes to connecting, becoming one, parenting, and running the house. Mom and Dad have to do talk all the time and sometimes we are not perfect in having one will—even less so when it comes to do the Father's will.
However, the Father, Son, and Spirit communicate perfectly, know perfectly, love perfectly, and therefore have one perfect will. They never have to argue, fight, or disagree. They can certainly discuss anything (and Father and Son do) but they all want the same thing and want the same way to get to that same thing. (This is most evident by the cross of Christ.)