not all uchiha have a 3rd power... i think this only applies for amatersu, tsukoyomi, susano
shisui is a good example he only ever had 1 MS tech
obito only has 1 tech but both eyes do it differently
Kakashi's eye - Long range - offensive
Obito's eye - Short range - defensive
Shisui was not a good model being that they only talk about one MS tech because its the strongest not because that's all he had. I don't believe in the manga it is mentioned that the people with Susano had susano BEFORE the first time they use it. I don't think he was saying that not all Uchiha have the third power i think he was saying that more along the lines that its rare that they develop it that far, being that only a handful of them awaken MS (which is strange being that they would have experienced so much in loss in Madara's youth you'd think half the clan would have one)