IMHO, If we look this in terms of cost , it will be a lot cheaper for you to spend every month the price of 6gallons than to change the entire gas tank volume, even if it would be possible (which it might be or might not be).

 

How far does your '12 3 go on a tank of gas? The '12 3 was rated at 18-28 (261-406 miles a tank) mpg city. The '15 3 was rated at 25-30 (330-396 miles a tank) mpg city. On trips, I have gotten over 590 miles per tank of gas with my '15 3MOE.

 

I have a 2014 EU diesel so it may be different, but I believe it is still a 14.5 US gallon tank and they have changed the "empty" from 0.5 to 1.3 to give a bigger safety margin.
If you were going to go to the expense of changing the tank it might be worth testing my theory by running yours down to empty then draining to see if there is any safety margin? I don't suggest carrying a can and letting the car run out as you might not be in a safe place to stop and you could get any rubbish from the bottom of the tank in the pump/system.