In all Residence Districts, a non-conforming use of land with minor improvements that is listed in Use Groups IV, VI, VII, VIII, IX or X and not permitted as-of-right within C7 Districts, may be continued for three years after December 15, 1961, or such later date that the use becomes non-conforming, provided that after the expiration of that period such non-conforming use shall terminate, and thereafter such land shall be used only for a conforming use.