I didn't speak until I was three also. But - just like Einstein - when I did start talking, I went straight into full sentences.
And, secondly, the thing about Einstein not doing well at school is a myth because the school he went to changed their grading system after he left so that scores of 1 or 2 (which were originally the best grades) became the lower end of the scale. So someone saw his report card and misinterpreted it.
As told
here.