Loudest Piezo with low voltage and current?
When a piezo buzzer is not delivering enough volume, the most like root cause is one of the following:


Input signal is too weak A signal may need an amplification stage before the piezo (even a venerable LM386 or similar would do)
Response is poor for input signal frequency piezo buzzers typically work best over quite a limited band, say 2-5kHz with a peak around 4kHz.
The particular piezo component is not up to the job 3V may be at the lower end of what they need to drive a decent sound. Use a test signal to verify it can actually generate the volume you are after. It might just be a dud unit, or maybe another brand/model would work better. Check the datasheet if possible (although I know for piezo buzzers they can be hard to find and verify they apply to the unit in your hand!).  


NB: This Peizoelectric Audio Components Application Manual by Murata is one of a few that include a some good details of how to drive peizos and considerations that apply.