Most cheapie air horns are 30 amp, so your 3 amp wire will be no good especially over that distance, most of the 12V power supplies that Jaycar sell dont have enough amps to power anything more than 10 amps, most of the people that work for Jaycar would not have a clue about electronics so its not worth asking them unless you jag a actual electronic enthusiast that works for them (few and far between).
You would need a power supply 30 amps or more, this one would do the job but you would need to upgrade the wire feeding the horn as the initial start up of the horn would pull the full 30 amps,
https://www.jaycar.com.au/13-8v-40a-...upply/p/MP3089
I find Australia post sticking most of the junk mail I get in my letter box.