I would know if there is a way to determine if the application connected using 3G or WIFI network.
You're not going to be able to determine this on the server side only. The only thing you can check is the HTTP user agent, which will help you figure out the device. However a connection is just a connection, there's no way to determine how it is connected without the device telling you. If you are developing a local mobile application then you can either do different logic on the device or send additional information about the device's connection.