If you look closely at the photo of the adaptor, you will see inside it a piece of metal. The opening on the AFM is rectangular, and smaller than 3". That's tha correct adaptor shown.
Whatever pipe you do from the throttle body to AFM, if its 3", fine. I would suggest wrapping the tube with insulating tape, as it is directly ove the exhaust. (If you leave the factory stuff, no wrap needed).
Just be sure to plumb in fresh air from outside the hood for the cold air intake.
Some people have used tubing from the AFM to run the filter out front, under the headlight, but you risk getting water sucked in, and blowing the motor. (I personally had a 2000 neon put a rod thru the block because the intake (factory setup) picked up the air down at the bottom of the bumper).
The puddle was only 14" deep.:icon_evil