Is possible to disable 3G connection in iOS and Android by Unity App?

Definitely not on iOS and a big probably not on Android. That's only because that's something the user should be able to control on their own and it's very easy to turn off. Apple doesn't like giving access to anything like that and I'm assuming you can't do it on Android either. I'm not sure for what reason you would ever need to but finding an alternative would be a different question, and if you need help with that feel free to ask.