The Catechism is where Catholics go to learn about the "central mystery of Christian faith and life."
234 The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith". The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men "and reconciles and unites with himself those who turn away from sin". - CCCMany Christians do not accept this teaching, other faiths reject it entirely.
237 The Trinity is a mystery of faith in the strict sense, one of the "mysteries that are hidden in God, which can never be known unless they are revealed by God". To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel's faith before the Incarnation of God's Son and the sending of the Holy Spirit. - CCCIslam rejects and condemns the doctrine entirely, which may explain why many Christians refuse to believe Muslims actually believe in the same God. I don't know about that however.
They say: "(Allah) Most Gracious has begotten a son!" Indeed ye have put forth a thing most monstrous! At it the skies are ready to burst, the earth to split asunder, and the mountains to fall down in utter ruin, that they should invoke a son for (Allah) Most Gracious. For it is not consonant with the majesty of (Allah) Most Gracious that He should beget a son. Not one of the beings in the heavens and the earth but must come to (Allah) Most Gracious as a servant. - Qur'anMormons and Jehovah Witnesses reject the Trinity, as do Seventh Day Adventists - these sects are popularly considered to be Christian, though they reject this fundamental teaching.
I like to think about the Blessed Trinity, to ponder the Indwelling - I like to wonder - in awe ...
266 "Now this is the Catholic faith: We worship one God in the Trinity and the Trinity in unity, without either confusing the persons or dividing the substance; for the person of the Father is one, the Son's is another, the Holy Spirit's another; but the Godhead of the Father, Son and Holy Spirit is one, their glory equal, their majesty coeternal" (Athanasian Creed: DS 75; ND 16). - CCC