Levi; He would immediately give however the guy was a nasty glare , if the guy didn't get the memo he would go up to him and make him run 55 laps around the castle.
Eren; This kid would go titan mood on whoever dared flirt with his everything. He wouldn't glare instead he would just walk up to the guy , and punch him when he got his anger out he would walk up to you and give a hard kiss mumbling a "mine" in your lips.
Erwin; He would be calm. Erwin doesn't need to be worried, he knows that you love him and would never leave him for another guy, so when he see's another guy flirting with you he would go up to you and give a kiss in the cheek , and give the guy a smile. Being pretty confident with your guy's relationship.
Armin; The moment he would see a guy trying to flirt with you he would panic. You being really beautiful would scare making him think that he's not enough for you. He would want to go up to the guy and tell him off but he would back off trying not to start something he knows he wouldn't win ,so he would back off to see what you did. He would be extremely happy when he saw you shake your head and scowl the guy then pointing at him with a smile on your face. Growing a bit confident he would walk up to you a give you a small peck on the lips totally forgetting about the guy.