my left eye has been twitching for the last 3 days?  What does this
mean and how can prevent it?

It's a sign that you are under too much stress. You can make an effort
to relax more, but it may take awhile to go away.

Here's an answer I found.  I also agree with stanmartin that it is due
to stress.  Twitching of the eyelid is actually very common, they are
usually temporary and harmless.  The medical name for this kind of
twitching is ocular myokymia.  The twitching is also due to fatigue,
lack of sleep, too much caffiene, or too much stress.  I have read
that if you gently message your eye, it will relieve the symptoms. 
The twitching should disapeer once you get some more sleep in.

It's likely to also be a magnesium deficiency. You may find your are
having other muscle twitches in the legs or elsewhere. Try taking a
magnesium supplement. Another magnesium defiency symptom is major
chocolate cravings.

I had the same problem for months. I took vitamin supplements
everyday. It just went away on it's own. Not a darn thing I could do
would get rid of it, and I wasn't under any stress. It comes back
every once in a while. I eat a horrible diet of Taco Bell and Pizza.