not feeling Tyrion as the third head. he's a Lannister. the special thing about the Targaryens is that theyre into dragons and shit, that 3rd spot belongs to Young Griff... or fuuck, go crazy and have Jon and Daenerys make a super magical dragon baby. i don't know. it just has to be a Targaryen.
it ruins the whole point of everyone's lineages and sigils if GRRM is like, "its okay, you can totally switch sigils because you're a favorite character."