The Islamic Revelation is the Revelation of the Unity of God, and this means that the Islamic understanding of God is on a plane that transcends dualism of any kind, which is to say it enters into the metaphysical, and not merely the ontological perspective. The Christian view, however, emphasizes the ontological perspective and remains there, at the level of Being, which is where theology develops itself.
What in Christianity is called the Holy Spirit is somewhat the equivalent of the Hindu Buddhi, or the Divine Intellect reflected in the order of manifestation. When Christians deify Buddhi, a part of “creation,” with God, they are committing what for Islam is an “association” (shirk) of the created with the uncreated God.
Further, Islam would really have no problem accepted that the idea of God comprises a ternary aspect, but they cannot accept the Christian insistence that this same idea is reducible to the Trinity, as if in an absolute sense, since any notion in which God is not an Absolute Unity is a relative understanding and Islam cannot condone ascribing relativity to the Absolute.
Divinity is both personal and impersonal, although this is hard to comprehend when one has been mentally formed by a way of thinking that only acknowledges the personal God. To explain it one way, we could say that God is “personal” in each particular Revelation, and is rightly seen as such within the confines of that Revelation; but God is “impersonal” or “suprapersonal” when seen as the principle of all religious forms without prejudice.
The distinction between the personal and the suprapersonal, much like that between the individual and the supra-individual orders, may be extremely difficult to grasp. Be patient with yourself and return to helpful texts again and again, and turn to meditation on the appropriate symbols. This will help you make progress.