Here the 1852 LDS edition omitted the repeated indefinite article a in the phrase “a great and a marvelous work”. The 1920 LDS edition restored it to the LDS text (although normally other losses of repeated a ’s have not been restored in the LDS text). For further discussion of the repeated a in the phrase “a great and (a) marvelous work”, see under 1 Nephi 14:7. The critical text will, for each case of the repeated a, follow the earliest textual sources.
Summary: Maintain in 3 Nephi 21:9 the repeated a in “a great and a marvelous work”, the reading of the earliest textual sources.