well as with any audible alarm and employees disconnecting them, the 0 tolerance rule being enforced usually fixes that if the employer will actually enforce the rule.
Usually when it is explained to them that this is a 'safety device' and that it should NEVER be tampered with they usually get the picture and do enforce that rule ;o) Discovering the culprit unhooking them and giving him a stern corrective action like a short vacation or in some cases termination of employment usually gets the message across to anyone else wanting to disable these devices ;o)

now as for the 3 wire connection?
my guess is that that 3rd wire is either a sensing wire or a signal wire turning it on.
Some of these devices with the 3 wire connection have a neg, pos and signal wire.
As long as you understand the circuit and know what it's function is even wiring in a 2 wire alarm should not be a problem unless it is directly integrated with the ecm, then you would need the 3 wire alarm.