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.

I do not know as i always just replaced them with a 2 wire species as i described.

The back up alarm on these Nissans is a 3 wire alarm. What is the 3rd wire for and where would I get a 3 wire alarm?

I have never been a fan of the variable db alarms as you are restricted to mounting options and install usually pointed towards rear and approx 4 ft off floor. This usually requires mounting on OHG post and wires are easily to be pulled out by operators that don't like noise.

well the one thing about those ecco smart alarms i've found is you have to be particular where you mount them. Putting them under the floor plate or in any enclosed area causes them to not work properly. They tend to be erratic with the variable noise level detection and tend to want to work at full db level all the time or vary because of the noise echo they detect from being in that enclosed area.
They are sensitive to sound so you would want to mount them in an open area not pointing towards any barriers or noisy components like the engine.
Generally they are used mounted inside the counterweight pointing outwards away from the truck.

Those alarms have a diode built in. Be sure your polarity is correct. Best i can remember i used to install the brand of alarm you mentioned next to the horn and ran a jumper from horn negative to b/u negative post then picked up the positive from reverse directional switch leg. These were a 97 or 97 db. Find the one that has the lowest operational amp draw. The oem alarms are a 3 wire set up and a pain to get at as you have to remove the battery. With with it mounted underneath floor pan, it is more protected and less likely of the operators pulling the wires to "hush" it.

try googling ecco alarms
they have many 'smart type' alarms that are multivolt capable, very versatile and come in different db levels.

check em out, they work very well.