There should be two holes in the front bumper support for the horns to fit into. The new horns will have studs on them so all you need is a couple nuts to mount the horns.

As for why the vehcile needs three horns, I cant think of any reason to have two horns in the front each with a differetn tone. I think part of the reason they used different horns for the alarm is because you can cut the wires for the front horns without triggereing the alarm. The alarm horn is located inside the vehicle so you cant shut it off. It has a battery in case the battery power is cut.

 

thief will usually cut the horn wires when trying to steal to make the alarm shut off. Putting it in the rear quarter panel makes it inacessible from the outside unlike a front bumper and if you cut the horn you still have the siren for the alarm connected.

 

I think not having a horn is illigal. and i try not to use it, but thats kinda like saying id rather avoid the accident then wear the seatbelt. its a safty feature. and its also fun to mess with people....lol