talk to the Father? And the Father talk to the Son? For example when Jesus was baptized, the Father in heaven was talking to the Son on the earth, and the Holy Spirit was descending on the Son like a dove. So clearly they are separate beings. Now possibly the HS is not a being, but just God's spirit, but the Son is clearly a separate being from God the Father. We don't know if the Son is God too, but he is clearly a different being from the Father. A different person, created by the Father.
On the other hand, Melchizedek is not created, but eternal. So he always existed. He too might be God. But he is not the Son, see Heb. 7:3. And he is not the Father, see Heb. 7:1.
As you said, everything is possible with God. So he could be several persons. He can create more persons within himself. So he might be now many persons. We cannot know. We know God is complex, but we don't know if the complexity involves being more than one person. If the Holy Spirit is a person within the Godhead, then the seven spirits of God mentioned in Revelation can be additional persons within the Godhead. But we don't know if they too are created. Maybe God is making himself more and more complex. But of course nobody can know everything about God. But we do know that the Father, the Son, and Melchizedek are 3 distinct persons. The idea that God is one person in 3 modes as the modalists, like the Oneness Pentecostals, claim, cannot be correct.