Why not buy a decent high power antenna? it'll be watertight also if you get one for outdoor usage, and we tested one ourselves here near us, gets a 1Km range by default in the city with it being mounted inside,so thats a good .... 0.8 miles or so. nowhere near your 3miles needed/wanted but getting 3 miles on a normal lowpower router is gonna be a PAIN, you either need to use amps, which create more heat, more power usage, or get yourself some decent kit.