Have a look at the publisher's site (the main book is Lingua Latina: Part I, Familia Romana
, the first one on the list). They have links to sample pages:
Also, search these forums for "Lingua Latina" and/or "Orberg" (the author) and you'll find a lot of information about that book (and read from a lot of its devoted followers).
Third grade is tough, I have a daughter starting it this year, and he might find the book to be a little challenging, but I still think it is by far the best choice (certainly would be for a 6th or higher grader).
Another option to consider, though not discussed here much, is the Minimus series. You can find more information about it here:
http://www.cambridge.org/uk/catalogue/c ... 0521659604
I'd worry that it might be too much to the other extreme (i.e., too easy) but I haven't seen the actual books or investigated it too much. Maybe someone else will comment on it/them.