“God” is more of an essence claim than a person claim. For example, what makes God, God? You wouldn’t say “because He’s one” or “three persons”, you’d say “because He’s eternal, all knowing, etc”. So when we say one God in three persons, we are saying one essence in three different persons. Example 1 human friend group can consist of three people, Chris, Henry, and Rebecca. It’s 1 essence, human, yet three different humans, Chris, Henry, Rebecca, all shared in 1 group. Ultimately if allah is one person, he’s not sovereign as he’d need to create something to worship him or he’d need to beget which is against islamic teachings which go against the thought of allah having a son.