He wrote, "im15 now in 3rd grade i had this test for add but they found nothing"

What I believe he meant to say was, "I'm 15 now. When I was in the 3rd grade I was given a test for ADD, which turned out negative."

I do not think that a person of high sophomore school age would be placed in the third grade, even before the "social promotion" ideas were adopted. This is not to say that some of them don't belong in a class at that level, though; third grade should be available only after they have mastered the first and second grade educational materials.