[Preorbtat]

When I medidate in my 3rd eye for enough long, consecutive flashes of white light start...
What does it mean?

At the begining, I thought it was my 3rd eye, with its ability to catch high frequency light such as auras, that was seeing the "off/on" normal behaviour of light bulbs. Now I think it's a little exagerated, plus the strobe light effect happens sometimes in daylight. (less frequently though, in this case)

is it normal? is it a good sign of 3rd eye developement?

[Dyerryjex]

It's normal.
I believe it's covered in the AD pedia, under Energy Work- don't know if that's what you're looking for- but it happens to meditators.
It has been described to be caused by an effect of the optic nerve, but I don't know if this is true.

[Preorbtat]

Does it have any negative effetcs on the eyes? should I worry?
is it a good sign or a warning that I'm doing too much energy work on that part of my E-body?

[Dyerryjex]

It gives me headaches, so it can't be all good, so to speak.
All energy work should not be done if it causes pain, and third eye work is more so.
Robert Bruce recommends that third eye work should be the last thing you do- tertiary centers, secondary centers, and when finally getting into primary centers (chakras) a lot of base center work be done before going up the body.
Sometimes you can get a third eye strobe from working on other parts of the energy body, because everything is interconnected. But if you have any doubts, do lots of grounding work to keep balance, and if there is any pain associated with third eye work don't do it- the higher chakras are very sensitive and even working on lower chakras will indirectly stimulate them.
If no pain is felt then there should be no worries, provided your energy work is balanced.