Order of the Stick (http://www.giantitp.com/cgi-bin/GiantITP/ootscript) answered this one... (though I'm not going to hunt down the particular strip just now)

Give the goat behind door 1 a good swift kick. If it yells that it hurt, that is the goat that tells the truth, open door 2. If it yells that it didn't hurt, it is the goat that lies, open door 3, and maybe it will be a car, or the 50% goat. In either case, you may have just kicked the 50% goat, in which case there is no car, and it doesn't matter which door you open.

If the goat doesn't yell, kick harder.