Hi
There are many smoke alarms out there (as posted) operating on 12v or you
can buy a 240v unit with relay from most good diy outlets or if you are near
an alarm supplier see if they do an adapter unit .
As for the 3 wire to the piezo I can only think it is for the test button as
some piezo sounders are dual voltage amd have 3 wires (com ,low,high).
HTH
CJ