Here is an interesting answer to the question of whether the “Let us” of Genesis 1:26 is referring to the Trinity. In The King in His Beauty: A Biblical Theology of the Old and New Testaments (Baker, 2013), New Testament scholar Tom Schreiner (Southern Seminary) argues that (1) it is doubtful that the author of Genesis was specifically thinking about the Trinity when he used this expression, (2) it is doubtful that the earliest Israelites read it this way, but (3) it should still be understood as a reference to the Trinity when it is read as part of the whole canon of Scripture.
Here is his explanation:
Recent developments in hermeneutics, however, have rightly corrected an overemphasis on authorial intent. Interpreters of sacred Scripture must also consider the canonical shape of the Scriptures as whole, which is to say that we must also take into account the divine author of Scripture. Nor does appeal to a divine author open the door to arbitrariness or subjectivity, for the meaning of the divine author is communicated through the words and canon of Scripture. It is not the product of human creativity but is textually located and circumscribed.
A canonical approach supports a trinitarian reading, which is suggested by the actual words of the text and confirmed by the entire canon. The Spirit’s role in creation is signified by his “hovering over the face of the waters” (Gen. 1:2). Psalm 33:6 . . . probably alludes to the work of the Spirit, for the word “breath” is the word used for “Spirit” (rûaḥ), and hence here the writer attributes the creation of the world to the Spirit.
In light of the NT revelation on the divinity of the Spirit, it is warranted to see the Spirit as creator. The Son’s role as creator is even clearer from a canonical perspective. John’s Gospel commences, “In the beginning” (John 1:1), an unmistakable allusion to Gen. 1:1. Another allusion to Genesis immediately surfaces, for John 1:3 speaks of the role of the “Word” in the beginning, claiming that “all things were made” by the one who is the “Word.” Hence, the “Word” that spoke creation into existence (Gen. 1:3, 6, 9, 11, 14, 20, 24, 26) is identified as the Son of God—Jesus the Christ (John 1:14).
Hence, from a canonical perspective, the “let us” in Gen. 1:26 should be understood as a reference to the Trinity.