Possible Clues:

• Three-in-a-row game
• Game with losing lines in 18-A, 28-A and 48-A
• Two-person pencil-and-paper game
• Game of X's and O's
• Pencil game that hints at this puzzle's theme, found in the first and last letters of 18-, 25- and 43-Across
• Pencil game that often has no winner
• "Hollywood Squares" basis
• Game on a three-by-three grid
• Winning line
• Game with a grid