it's a dual output siren. probably warble and steady, or rise/fall and temporal 3.
one terminal is a ground. attach the black to that. the other two terminals are the tone selection. put the positive wire on whatever tone selection you want, and leave the remaining one not connected to anything.
reason for this is so that the device can be used for burlgary and fire indications. fire warning devices need to produce a specific tone cycle called "temporal three". on some systems, warble is burglary and steady is fire. other panels code the sounder output pulses if it's a fire alarm. each alarm panel is different, but sirens are a universal deal. an ademco siren can be used on a radionics or DSC panel or anything with a 12v output.
it's a matter of preference in your case. warble or steady. what type of siren are you using?