i have a K&N air filter in my car atm, would there be any bennifits with using a 3" AIR INTAKE FILTER? would you need to get the ECU re-Tuned for it?

By the 3" air filter I'm guessing your talking about a pod filter? If you are, the benefits are minimal. And no there should be no reason I can think of that you would have to re-tune your ECU.

Unless, you were to put the pod filter inside the original box the square filter is currently, then still the benefits are minimal, but yes there are some benefits.