Looking for a young-adult science fiction author?

I believe it is the same writer,

the first book was about a boy who was trained from birth, and genetically altered to meet other cities in a contest to determine who would win control of the only island free from disease in the world. They lived in a dome city..

the second book involved a boy with a mean girl cousin who finds some sort of liquid that gives him super powers, the girl cousin finds it as well and they fight, or something, I can't remember

the third book involved a boy who played a board game about space, but the people he was playing with turned out to be aliens.

The author's last name deffinately started with an "S" it was sline, or sloan or something like that.

Any help? I will add more details if I can remember.

cole i has been correct on books two and three, but book one is probably by a different author then.

The main character is treated as "special" isolated, but treated with all the best things. It opens with him being trained via laser reflex training. The final conflict involves maneuvering small soldiers on a map.

The second book sounds like a book I read called Others See Us by William Sleator
http://www.amazon.com/Others-See-Us-Will...

The third I believe is Interstellar Pig
http://www.amazon.com/Interstellar-Pig-W...
which has a sequal call Parasite Pig
http://www.amazon.com/Parasite-Pig-Willi...

I don't think the first one is the same author. I've gone through summaries of his books and nothing is popping up.
Gothca! The books were probably shelved near each other. The author is Gloria Skurzynski.
http://www.amazon.com/Virtual-War-Chrono...

hello. maybe you're meaning Orson Scott Card, 'Ender's Game.' a very different kind of Science Fiction Novel.

The last one sounds like it might be Orson Scott Card, but in "Ender's Game" it wasn't a boardgame, it was a "simulation," supposedly against a computer that was training him to be come a fleet admiral, but he was actually directing a real military campaign without knowing it, against a race of aliens.
So it could be Orson Scott Card, but that doesn't quite match your description of the third book.

I don't recall any books similar at all to the other two, though.

could it be R.L. Stine? He wrote those books called "Goosebumps"

I agree with cole i