It isn't going to an effective one at all if it is mounted right out in the open like that.
No alarm worth a damn that I have ever seen has just 3 wires. You need wires for the power, ground, triggers, door locks, relays, siren, acc (fuel/starter cut off, window/sunroof close, etc), led, etc. There is usually a huge bundle of wires, relays, sensors, etc with the good ones.
To make the sound you are going to need a siren. That box isn't going to magically make noise.
There must be something that I am missing in communication here.