When You add Uchiha to Senju you equal one Uzumaki. Madara said the Bloodline of Uchiha and Senju had thine out. Uchiha + Senju = Uzumaki closer to the source SO6P.
so what you are saying is that it's your
theory that the SO6P was an uzumaki, okey, I respect that but then say it that way and don't go saying things like "Actually the SO6P was the first Uzumaki"
and I agrea with apple pie of doom, if it's not stated in the manga it's still theory.
in my oppinion the uzumaki are just decendants of the senju bloodline. kushina did say that the uzumaki are distant blood relatives (on chapter 500 page 3) of the senju, so to me that sounds like the uzumaki evolved from the senju. so to my oppinion your litle math work there doesn't fit, but it's just my oppinion.
and just to go on toppic, I don't think there is a third son, kishi would have mentioned it by now, it would be to important.