Why do Catholics disagree that God is not 3 in 1?

Roman Catholic AnswerThey say they believe in one God because they do believe in one God. The fact that there are three persons in one God does NOT mean that they believe in three Gods, it means that they believe in three persons in the one God. Look at it this way, a human person has a Body and a Spirit - but he is not two things, he is only one thing - a human person, but he HAS two things - a body AND a spirit, together they make one thing. The analogy is not perfect, but with God it is the other way around, He doesn't have a body, only a spirit - but there is only one spirit, the Divine Spirit, that we call God. That spirit has Three Divine Persons, but they are all one God, not three. It is a mystery that is beyond our understanding, but we accept it because it has been revealed to us. The important thing to understand is that we only believe in one God and there can only be one God. By definition, if there were more than one Gods, they wouldn't be God.