Illustrator switches levels when exporting

Hello everyone,

I have got a problem which drives me crazy and i hope you are able to help me.

I have developed a brand logo which consists of three bis letters and a black 3-D shadow.

So, my problem is that if i try to export this file to png or pdf or whatever, it changed the levels i.e. it brings the black shadow to the front.

I have no idea what i did wrong and how to solve this problem. Hopefully you can help. Thank you