Who are other targaryens?
a) Jon snow b) Tyrion Lannister c) Robert's bastard-Gendry
a) As everybody wanted this, Jon should return from dead when his body been burned. this will prove he is a targaryen and fire cant kill him. But targaryens are only fire resisters and can die. so what if the dead cant come back? And also in s1e1, when king robert asks ned about his bastard's mother name, he mentioned Wylla. So Jon is not a targaryen. But also what if he is the son of rheagar and lyanna? ned might not want to disclose it to robert as he would feel in a wrong way. By this theory, Jon is also a targaryen. Also he possess the valyrian steel.
b) As Tyrion joined Daenerys, he is now in targaryen side. But he might also be a real targaryen but not as a targaryen's bastard. What if Tywin was not real lannister? My thought is tywin might be a bastard of some targaryen. As per the targaryen habitual, tywin has 3 children, brother sister incest, cersei has three children and they possess valyrian steel. the show will turn as all remaining lannisters will die except tyrion and he will be also a targaryen by blood.
c) As known, the baratheons have targaryen blood and the remaining baratheon is Gendry(robert's bastard). Remember when Melisandre told Gendry has king's blood and he will be needed. this highlight that he will rise later may be as a targaryen.
So from these three guys, the second and third targaryen will rise and fight along with Daenarys all three in three dragons.