for onew dont use PVC. it is way to thin and will not form a nice bend. i would use ABS piping which is 4x as thick and will take much more heat. when u heat it to bend it your gonna wannt heat a much larger section of the pipe to get a uniform bend. try a 5 gallon pail or a small tree. you dont want to make too sharp of a bend either. hope that helps.
Also, you will be losing performance with pvc pipe.
On intake tubing, the 3" diameter is measured from the inside of the pipe. On pvc and other pluming pipes, the diameters are measured from one outside of the pipe to the other. So in between those 3 inches, you have about a 1/4" - 1/2" of the tube, so you will not have a 3" diameter with pvc.
dont think a heat gun gets hot enough, this is a bad idea all around, PVC wont stand engine temps and will become misshapen from the heat, there are other kinds of plumbing pipe that will work but still i would rather get bent metal, or pre cast plastic