Can I use european headphones on american iphone?

I want to buy some headphones here in germany, will they work on my american iphone, pc and mac?

I would say yes with about 95% certainty.

iPhones, PC's and Mac's are all sold internationally. So are almost all other computer and audio gadgets. Every one I have ever come across all comply to the same standards of tip-ring-sleeve.

The only reason I won't say 100% is because every so often (much less lately) some company tries to go with their own proprietary system in order to rope you into exclusively using their products only. Sony was famous for this but I think they learned the lesson that it costs them customers in the end.

Why not just ask the sales rep there?

All headphones work. As long as they are not the headset with Mic for voice conversation. The voice conversation headsets differ from brand to brand.