Just signed up managing agent for first CIP - at 5% + GST.
Having spoken to other agents I've seen anywhere between 4% to 7% with 4 - 5% the norm. Have not seen 3%, but to be honest this is our first CIP and we didnt speak with that many managers.
Obviously you also need to take into account the re-letting fee (half a months rent for me), or completely new letting (one months rent).
My other 2c is you'd want somebody who will go in and bat for you at rental review times, and who talks up rents in the area and has a monopoly in the area.
Good luck with it!!