This is usually brought up in debates in which the trinity (or who Jesus is) is defended or attacked by one side of an argument.
The fact of the matter is that Unitarianism must be assumed unless evidence otherwise is given. If God can be a being shared by three persons, such as in the Trinity of Christianity, then there can be no basis for puting any limitations on the nunmber of persons who share eternal
The doctrine of the Trinity is simply that there is one eternal being of God - indivisible, infinite. This one being of God is shared by three co-equal, co-eternal persons, the Father, the Son, and the Spirit.