There can be no doctrine of the Trinity without a prior doctrine of divine simplicity.
The doctrine of divine simplicity was not exegetically derived, it is a doctrine of natural theology. It is a philosophical assumption which we use to make sense of Scripture, as the law of noncontradition is.
Interestingly enough, Plotinus worshiped a divine simplicity; this is the logical conclusion of the Platonic trajectory.
This is the Platonic trajectory:
1 The imaginary encounter between Socrates and Parmenides in the Platonic dialogue Parmenides shows two things. The first is that Plato favors the Parmenidean one over the Heraclitean many. The second is that it is demonstrated that forms are of different kinds: not all forms are equal.
2 Plato develops his doctrine of forms over the years. They exist in ambiguous relationship to God because he is not interested in the question of God, he is not even interested in a system as such. There is, however, an emerging hierarchy of forms.
3 The relationship of the the forms to God concerned Middle Platonists, who were beginning to feel the influence of Judaism’s monotheism. They move the forms into the mind of God.
4 Plotinus sees that the forms cannot be in the mind of God. He posits a form of the form, a radically transcendent and ultimate One, or the Good, who is the absolute principle of prior simplicity.
The arguments for this radical, prior simplicity, this from which all forms derive though in it they are not many, not separable, not even distinguishable, are assimilated by Christian theologians and assumed when reading Scripture.
In Eriugena, the relation of God and Creation is introduced as a way to set a discontinuity between the divine simplicity of Plotinus, and the realm of forms. What was for Plotinus Nous, for Eriugena is the Image of God, that is the archetype of Man, in whom all of creation finds its forms, and who is not God, but who is informed by that which is form of the forms.