I havn't read a whole deal on this, and I can't quote anyone, but I think the numbering system is arbitrary. More used to give us an idea of what is going on, rather than to say that the Son is Second to the Father, etc.
I think the numbering was put there to better describe who does what. The Son was begotten of the father, therefore Second, and the Spirit proceeds from the Father, "used" by the Son, therefore third.
I have no idea if I answered any questions. My statements were by no means official. Just some random thoughts...
