What is the minimum necessary description of God?

First, all Catholics, Orthodox and Protestants believe in the triune Godhead - that the Creator is one God: the Father and the Son sharing the one, divine nature, with absolute unity of the Spirit in that one nature. The Reformers never had any issue with Catholics about the Trinity doctrine. On that, we are agreed.

The Jewish faith is also monotheistic, but they abhor the Trinity doctrine, mainly because they reject Jesus Christ as the Son of God, the Messiah. Yet there is no doubt but that the one God of the Shema declaration of Deuteronomy 6:4-9 was utterly believed by Jesus (Mark 12:29-31) and by all his followers. They were all referring to the same, one God. Had the Jewish people of Jesus' day been prepared by John the Baptist with his baptism of repentance, they would have recognised Jesus as the Messiah and so would have gone on to understand God as their Father in heaven, the way Jesus taught, where God spoke three times from heaven to declare Jesus his unique Son.

This brings us to the other monotheistic faith you mention, Islam. (I don't know why you only mention the Sunni branch when the Shia branch also believes in one God.) However, the waters had become muddied by the time of Muhammad. From what he had heard of Christianity in his day (the 6th century A.D.) he misunderstood Jesus, and went along with what he had been told. He thought Christians "ascribed partners" to God, which is the worst sin in Islam. So, that Christian doctrine was condemned and Jesus said to be only one of many human prophets, even if a great one. His prehuman existence in the Godhead was denied, and they said he was not crucified nor resurrected and is not our intercessor. They claim that Islam represents "true Christianity". Their denial of the Christian concept of God (the Godhead) means that they reject the Christ of the Bible, sticking to the Christ of the Qur'an; the Jews reject Christ as the Messiah of their Hebrew scriptures.

That's the problem in a nutshell. But to ask "What's the minimum necessary description of God" requires a person to study everything Jesus ever said about the Father and of himself as the Son of God. That cannot be done here. Besides, Christianity is not about "the minimum" regarding God - it's about the maximum God has chosen to reveal about himself, culminating in Jesus being the only one who can bring us to the Father.

Ponder Jesus' prayer in John chapter 17 where he said that he would be glorified with the glory he had with the Father, before the world was. He also said that God had given him power over all flesh, that he should give eternal life to as many as God had given to him; that life eternal is knowing the only true God, and himself. It's not knowledge about God and Christ that results in eternal life. It's knowing them. And you cannot know the Father until you come to Christ. He reveals the Father. Equally, the Father reveals Christ, and the Holy Spirit operates in all of this, for he was sent into the world after Jesus' resurrection to indwell, teach and guide Christians (John 6:44, 65; 14:17, 26 & 16:13-14).

As everything in that last paragraph is denied by followers of Islam and of Judaism, it would seem that whatever they do know of the one God can never result in life eternal, which Christ grants. To know the one, true God, we must bend our knees in Jesus' name, the name that is above every name, the only name under heaven given to men whereby they can be saved (Acts 4:12 & Philippians 2:9) That is the starting point. Try to approach God any other way and there will be no grasp of who the one true God is (John 5:39-47).