I’m pretty sure there is no such thing as zeroing in ac-measurements. There is a button for that, but it just does not do anything.. Hopefully.
What would you except to happen when you zero your ac meter? Filter anything below your threshold out or get negative ac-voltages, if you find lower level of voltage than your zero-level?
There will always be some noise when measuring and your example of 3 milli-VAC seems to be a very low one.