The 3G cellular network offers a high speed connection to the internet. In return for providing this additional connectivity, 3G smartphones consumes a lot of energy. In some situations you might need to disable 3G on your iPhone to prolong the battery life.
How to disable 3G?
To deactivate 3G , you have to go into the Settings of your iPhone, in the " Cellular Network" section.
Turn off "Enable 3G".
How to disable 4G?
Since the advent of 4G , operators have changed their profile to indicate that 4G network is being used.
If your phone is connected to a network operator offering 4G , the "Enable 3G" option is replaced by "Enable 4G" option.
Even if your disable the 4G network, 3G remains active. If you then want to disable 3G too , turn off "Cellular Data".