I haven't myself noticed a chain of celebrities dying in threes. I guess it depends on what you term a "celebrity". In my view, having managed to get yourself noticed by a large number of people doesn't automatically qualify you as a person with gifts that should be "celebrated"...as many reality TV shows demonstrate quite clearly :)

Anyway, I can give you a metaphysical explanation for the old idea that "bad luck comes in threes".

I believe the ancient mystics used to call this The Law of The Triangle or The Mystical Triangle.

Consider some kind of force (physical or non-physical) being exerted. Let's label that first force as ONE.

Now in order for anything to change in this (physical or non-physical) reality, we must have a force TWO come along and interact with it. Having another ONE come along only adds to the existing ONE...so in this case ONE + ONE does not equal TWO :)

So, in our (physical or non-physical) reality, now we have... force ONE added to force TWO

Because we know that ONE must be different to TWO (from above), we must therefore get a creation of something new that we can now call THREE (an "effect" resulting from the two "causes").

From that creation of THREE we have now expanded our (physical or non-physical) reality

This is one of the reasons why the ancient mystics considered the number THREE to be so important.

So the mathematical/mystical equation that I'm expressing here is...

ONE + TWO = THREE

Yes, I know it's obvious. And even babies are probably taught that mathematical equation these days :)

But as you can see, there is a much deeper mystical concept behind the mathematics that virtually everyone overlooks.

The ancient mystics used to embody this concept in the form of Triangles where one corner of the triangle used to represent ONE, the second corner would represent TWO and the third corner would represent the newly created result... THREE.

Another way of referring to these mystical triangle groupings is as a Trinity

Here is one such famous Trinity...or famous to Christians at least :) ...

In this case, ignoring all the usual dogmatic associations, what we really have here is a diagram representing the creation of a human being.

We have ONE as "God", which is symbolic of the underlying universal consciousness that exists.

We have TWO as "Holy Spirit", a term which has been misinterpreted to mean ghost-like entity but which the ancient mystics used to use in reference to the energies that make up physical reality.

Now that leads us to the creation of THREE...Human Beings (Human Consciousness)...represented by the idea of the "Son"...labelled as Jesus ("The Christ Consciousness") on this diagram.

So what this diagram actually expresses is...

UNIVERSAL CONSCIOUSNESS + PHYSICAL REALITY = HUMAN CONSCIOUSNESS

To put it simply, if non-physical consciousnesses express themselves through a physical reality, the idea of a human being (a focused blending of physical and non-physical) will naturally result.

Now how do all of these ideas tie into the old "bad luck always comes in threes" notion, which seems to underly your question?

It comes from people noticing over time that once a ONE appears and a TWO is interacting with it then a THREE must surely result, even if it hasn't physically manifested yet.

In the case of that saying, people are noticing unwanted things as ONE and TWO (which will lead to an unwanted THREE), but it doesn't have to be that way.

There are a zillion different examples of these Triangle-like cause and effect relationships, and you'll start to see them everywhere once you realize they exist.

To take a modern-day dramatic political example from the Middle East, we have had a few times now...

ONE = Dictator in charge of a country

TWO = People angry at the dictator for being repressed

Now that we have a ONE and TWO interacting then, sooner or later, that THREE is coming....recently that THREE has appeared in the form of political uprisings in that region.

Another example to ponder:

ONE = Rigid systems of conformity throughout human societies

TWO = People beginning to reawaken to the fact that they are powerful multi-dimensional consciousnesses and rebelling against the rigidity

...Methinks there is a THREE on the way :)