This subtle but culturally significant detail is a brilliant example of how Tarantino uses cultural nuances to heighten the tension in the film. The "drei Gläser" ("three glasses") line is an ingenious but subtle sign to the audience that the jig is up. Hellstrom also displays that he is extremely good with German accents (the spy's accent seems pretty fake to him). Last summer while dining in Zermatt I asked my Swiss server where he thought I was from (we were, naturally, conversing in German). He guessed either Belgium or the Netherlands. Care to listen in?
Conversely, when I'm in Germany I'm often mistaken for a Swiss. I even knew someone in Basel who could tell what quarter of the city you were from based on your dialect!
Most Americans and Brits probably wouldn't notice the 3 finger motion because it seems so natural to us, plus they can't hear any differences in the spy's accent. The details and references are so specifically German that a native German had to have either written or advised the scene. When I tell you that the shot of the spy's hand holding up the wrong 3 fingers had me on the edge of my seat, I'm not joking. I have no sufficient words (in either English or German) to describe the dialogue in this scene. It is like a renaissance painting. It's a cinematic masterpiece.
Like probably 99 percent of people who go to the movies, I tend to watch films and enjoy them for what they are. But when you can see the details and the fine cultural nuances in the movie, a whole new perspective opens up. Which reminds me. When we read the Scriptures, we have to pay attention to the cultural and historical context of what we're reading. When you study any portion of God's word, make sure you study the background. Recreate in your mind the culture if you can. This will make the text come alive as never before.