Well ~actually «Su» can also mean "your" (singular) in a formal conversation, and "their". ● Tu casa → Your house [informal] ● Su casa → His/her/its house ● Su casa → Your house [formal] ● Su casa → Their house you gave some examples,I will correct them for you: I am your friend "Soy tu amiga" or "Soy su amiga". that's te correct way :) I hope I've helped you.

I forgot ... You can also use the plural way like: Tus amigos=you're friends(informal). Sus amigos(Formal).