To be honest, something about this movie I just don’t like. I just don’t know what it is. Doesn’t make sense to me because there are a lot of scenes, like this one, that I like.
As a German, I think this scene is overrated. Yes, it's true we use other fingers to show the number three, but if _only_ such a silly gesture gets the spy to expose himself, either the Germans in the movie are too stupid to recognize other signs, or the American spy is otherwise perfectly trained, but somehow no one in his years of training told him to use the German gesture … For the record: I'm a huge Quentin Tarantino fan.
The "3" gesture was just the final nail in the coffin. Hellstrom was already skeptical of Hicox because of his accent and his reluctancy to answer Wilhelm's question, but the "incorrect" gesture of the number three was "the straw that broke the camel's back" so to speak.
As a german i think this is one of Tarantinos best scenes. Tense, funny and written to the point. Also maybe you missed that the SS officer is only comming to the table becasue he is already suspicious because of the accent. Also, its a british spy, not american. And he trained for a few weeks tops. He is there becasue he can speak decent german and knows about the german film industry primarly, not because he is a super skilled sleeper agent.
