Doesn't the Trinitarian reading of Genesis 1:26 violate hermeneutical principles?

Why do Trinitarians rarely mention the hermeneutical principles which have led them to this belief? What are the hermeneutical principles that support the Trinitarian view? For example Genesis 1:26 "Let us make man" The common Trinitarian interpretation of this verse is that it proves the Old testament taught the Trinity doctrine. Yet it breaks a basic rule of Bible interpretation: "Try to find the original writer's intentions when he wrote the biblical text". From what I have researched, Trinitarians break this rule and several other basic rules of biblical interpretation.