Yes, thirds is a Scrabble word!

Noun

1. One of three equal parts of a divisible whole

2. The fielding position of the player on a baseball team who is stationed near the third of the bases in the infield (counting counterclockwise from home plate)

3. Following the second position in an ordering or series

4. The musical interval between one note and another three notes away from it

5. The third from the lowest forward ratio gear in the gear box of a motor vehicle

6. The base that must be touched third by a base runner in baseball