in Astrology, Destiny and the Wheel of Time - ch.1
To earn money one has to work, show valour, and perform efforts. A malefic in the 3rd house is preferred to a benefic generally for two reasons: first, a malefic here shows the will to do hard work, not soft work. The second reason is that the 3rd house is one of the trishadaya houses (3, 6, 11) which are described as evil houses. Malefics here become a negation of negation, or two negatives making a positive.
reprinted with permission