How Do You Know There Aren’t Three Distinct Divine Beings?

Questions about how we know that God is one being instead of three distinct divine beings in perfect unity, and why God would give notable miracles to Oneness Pentecostals if their theology is truly heretical.   I see real distinctions between the persons of the Trinity—Father, Son, and Spirit interacting, speaking, and acting in different roles—but what specifically makes God one being instead of three distinct divine beings in perfect unity? Why would God give such notable miracles to Oneness Pentecostals if their theology is heretical? I can’t help but wonder if they’re seeing this kind of power because they know who Jesus really is.