Foxi, if I install a CFW, play backup games offline and after that play a real game on PSN, can they ban me from PSN?
You shouldn't be playing on CFW using PSN unless you have some safety measures installed and you're 100% certain that they work. PSN is for OFW users only, at least until ODDE is released.
OK. And if I use the 3key?
3k3y, much like WODE is a drive emulator - the console has no way of determining whether it's reading a real disc or not, the ODDE has the keys of your original BluRay drive and the ISO is mounted as if it was a physical disc. There's literally no difference provided the dump is proper, so it's highly unlikely, if not impossible, to get banned when using one. That, and you only install the ODDE in CFW Mode - the moment it's installed and has the extracted keys, you can safetly install OFW and the ODDE will do the rest... so it's something to look forward to, yes.