The same folks who bring you the X-10 Spam will see you a variety of things that will do more or less what you describe.
The "three mile" part might push the envelope (you might have to use hi gain antennae at each end but for about $100 you can get a remote camera (RF link) that's about "web-cam" sized. For $3 or $4 hundred you can get a fancy colot camera that runs off a 9 volt battery and radiates on a UHf TV channel. Either of those could be powered by a rechangable battery on a solar cell.
I onced kicked around the idea of putting one of those TV transmitters on my dog so I can see where she goes when I let him out. But I decided that $400 was more than the knowledge was worth.
Have fun; tell us if you go through with it and what you see