How you doing is an informal greeting like How's it going. It is most commonly known as the signature pickup line of character Joey Tribbiani from the sitcom Friends.
BMS is an internet acronym for “broke my scale," used in support of a user, often a woman, posting a picture of herself and asking for a numerical rating from her online followers.
The Wonder Twins are a pair of DC Comics superheroes with interconnected, shape-shifting superpowers, sometimes playfully or mockingly alluded to for other closely bonded duos.
A Three Percenter is someone who advocates for a strict interpretation of the Second Amendment of the US constitution, strongly believing in armed rebellion against perceived government overreach, especially with respect to gun laws.