I think it is important for you to not focus solely on
becoming a "third grade teacher." When you begin your job
search, (not an easy task these days) you need to be open to
accepting a job at any grade level for which you are
qualified. Certainly seek out your preferred grade level, but
be willing to get experience at any level. Besides, even if
you get a job teaching 3rd grade, you never know when your
principal will decide to shuffle you around.