Possible Clues:

• Ross and backups
• Motown trio in the Rock & Roll Hall of Fame
• Famed Motown trio, with "the"
• The --, Motown group
• '60s group that had three women, with "The"
• After drink, rock group has drugs for another group
• The ____ are America’s most successful vocal group to date
• Diana Ross's group, with 'the' ... and a hint to the power group detectable in half of 21, 29, 37, and 46 Across
• Diana Ross' backup
• 33-Down's group, with 'the'