You need to have the 4G/LTE version of the 3rd generation iPad for have support for cellular LTE service. I can't tell from your post what version of the device you have. If you use "3G" for third gen, then we can't tell if you are saying you have the 3G (as in cellular) or 4G/LTE or what. If you don't have the black colored RF window along the top of the back of your iPad, you do not have the 4G/LTE version, you have the wifi-only version. Its is easy to tell because the WIFI only version doesn't have the RF window. The cellular and GPS antennas are behind the RF window.