MAC address is specific to network technologies that implement the IEEE 802 standard, such as ethernet. 3G is not an IEEE 802 network, so MAC address really doesnt apply.
That said, there should be very few reasons to use the MAC address in your app, unless youre writing a network driver or doing something specific with the TCP/IP network.
Often people reach for MAC address thinking that it represents a unique device id, when really you should use something like http://developer.android.com/reference/android/provider/Settings.Secure.html#ANDROID_ID to get a unique id that will work across all of the different hardware combinations for android, some of which might include no IEE 802 network whatsoever.