Christians, therefore, properly contend, from this testimony, that there exists a plurality of Persons in the Godhead. God summons no foreign counsellor; hence we infer that he finds within himself something distinct; as, in truth, his eternal wisdom and power reside within him.
John Calvin and John King, Commentary on the First Book of Moses Called Genesis, vol. 1 (Bellingham, WA: Logos Bible Software, 2010), 92–93.
Who will go for us? I am rather favourable to the opinion that this passage points to Three Persons in the Godhead, just as we elsewhere read, Let us create man in our likeness. (Gen. 1:26.) For God talks with himself, and in the plural number; and unquestionably he now holds a consultation with his eternal Wisdom and his eternal Power, that is, with the Son and the Holy Spirit.
John Calvin and William Pringle, Commentary on the Book of the Prophet Isaiah, vol. 1 (Bellingham, WA: Logos Bible Software, 2010), 213.
I know that many censorious persons laugh at us for deriving the distinction of the persons from Moses’ words, where he introduces God as speaking thus: “Let us make man in our own image” [Gen. 1:26]; yet pious readers see how uselessly and absurdly Moses would have introduced this conversation, so to speak, if not more than one person subsisted in the one God. It is certain that those whom the Father is addressing were uncreated; but there is nothing uncreated except God himself, and he is one. Now therefore unless they grant that the power of creating was common to the Father, the Son, and the Spirit, common also the authority to command, it will follow that God did not speak thus within himself, but addressed other outside artificers.
John Calvin, Institutes of the Christian Religion, ed. John T. McNeill, trans. Ford Lewis Battles, vol. 1, The Library of Christian Classics (Louisville, KY: Westminster John Knox Press, 2011), 152–153.