ac3 is really an output only format. As such you are unlikely to encounter a situation where this needs to be done unless you're trying to do something naughty like ripping DVDs.
There are a number of programs to do it, have a look around VCDHelp.com. I've had to do it once for legitimate reasons, only stereo ac3 to .wav and the results were reasonable. It's certainly not something you should have in any normal workflow as you may endup with serious loss of audio quality, much the same as dealing with mp3 audio.