In Matthew 5:6 [KJV], the sentence ends with “they shall be filled.” In 3 Nephi, “with the Holy Ghost” has been added.
The pure contrast is between those who hunger and those whose hunger would be sated. However, there is also the desire to move the meaning from simple desire for food to a more spiritual meaning. Therefore, those who “hunger and thirst after righteousness.” That hunger will also be sated, through the influence of the Holy Ghost. The addition adds to the parallelism.