I think fasting is one of those things thats very personal in wether there will be any benefits to it.
Before you ever do anything thats questionable, ask yourself the following
Is there anything sensible that gives the same results I am hoping to achieve?
Is the reason I am doing this because it promises faster results in an easier manner?
Fasting won't hurt you if done in moderation (I think when its not done in moderation its called Anorexia).
Our bodies can handle being deprived of fuel for a bit.
Why people would do that over just eating things like fresh/frozen fruits & veggies and lean protein I don't know.
I read all about fasting when I was starting to decide I had to take getting unfat seriously. I read all these good things about it, but everything seemed to mimic the others with only anecdotal evidence.
I think there are probably good reasons to fast - I've heard many times that food does taste better (which makes sense if you withhold sex for a while it feels better). If giving up "junk" foods is tough for you, maybe a total food abstinence would help. I don't know, BUT if a fast helps you personally then it works in some way.
Also, if you were eating lots of junk then you probably would feel better afterwards, I can't imagine you wouldn't (unless you went back to eating junk.)
Fasting for 3 days won't hurt you (probably) but there might be better options.