I know that everyone hates the wimpy horn that comes on the H3, but I was in Harbor Freight tools and came across the BadBoy Horn by WOLO. I paid about $38.00 from the store. It is an airhorn fully compact and attaches with the factory two wires and one bolt that goes in place of the factory hole. There is no drilling or hard wiring. The horn is a hell of a lot louder than the stock horn. They say it is around 118 Decibels. I think that this was one of the easiest installations I had. This is a very well made product. You can checkout WOLO's website and preview and listen to all the horns that they sell. The BadBoy horn comes either in the galvanized and black OR the chrome model. They also have them on ebay. Here is the link.
http://www.wolo-mfg.com/
Here is the install:
Step 1 - Remove the factory grill by unscrewing the 6 screws.
Step 2 - Unplug the factory horn wire and remove the factory horn from the mount (1 bolt).
Step 3 - Cut the end plug on the wire (cut a little bit back in case you ever need to reattach it later for the stock horn)
Step 4 - Crimp a female spade connector on each of the two wires.
Step 5 - Mount the BadBoy Horn on the factory mount with the supplied bolt and plug the wires into it.
Step 6 - Snap the factory grill back into place and screw it in.
Thats it you're done!