It seems to be superfluous to teach what cannot be known by natural reason. (A)
But it ought not to be said that the divine tradition of the Trinity is superfluous. (E)
Therefore the trinity of persons can be known by natural reason. (A)

AffIrmo
nEgO

Strictly logical form:

All things not known by natural reason are superfluous. (A)
But the trinity is not superfluous. (E)
∴ the trinity is known by natural reason. (A)