Derekh Hashem, Part Four, On the Daily Order of Prayer

Regarding the eighteen blessings: And behold, you should know that the types of supreme influence - under which all types of flow and their details are included - are three. And they are indicated in the three [first] letters of [His] name, blessed be He; and their binding together for the perfection of all creation is indicated by the last [letter in His name,] hay. And corresponding to them are the three descriptions [in the first blessing], "the Great, the Powerful and the Awesome." And that which brings them down as is fit is the merit of Abraham, Isaac and Jacob; whereas that which brings down the perfection that comes out of their connection is the merit of King David, who connects with the patriarchs and completes the refinement of Israel. And note that the first three blessings of the prayer were ordained to correspond to these three types [of influence]. So the supreme influence is brought down more generally through them. And afterwards, they are brought down in detail by the intermediate [blessings], according to the need. And through the last three, they are strengthened and internalized by those receiving them, through the thanksgiving which they give to Him. And this is the general refinement of all of prayer.