nth-child count all elements, and if the 4th element is a paragraph, apply the styling, otherwise don't. A better way to explain this, would be that p:first-child won't do anything, since the first element is a h3, and not a paragraph.
So, if all elements are counted, it should be nth-child(4), make sense?
That would work, but you forgot the slash in your h3 closing tag, which messes up the count
Please read this post about markdown so your code/indent will show.