What kind of resistor would I need to use to hook up a 9 volt battery to a 3 volt piezo buzzer?

Find out what current it takes. calculate the one resistor you need, R=6/current
If it's a 3V buzzer then use a par of AA batteries. Otherwise you would need a voltage regulator to take the 9V down to 3V. A pair of resistors in a voltage divider configuration may work but it's easier o use a different power source. But you can look up voltage divider calc online to help you figure out the resistors you'll need.
rickharris1 year ago
IF it is a buzzer you need to look at the specs and see what voltage it uses.

NOTE not all Piezo sounders are buzzers in their own right.

Often these are 5 volts. BUT look at the spec.