The Nano-10 digital inputs cannot directly handle a 3 VAC input. You will need some signal conditioning to convert this signal to something that the PLC can handle.
If you can provide a link to the data sheet for this sensor, I might be able to help you figure out what it will take to get this flow sensor working with the PLC.
Gary D___kinson
Substitute "ick" for "___" and you'll get my family name.