Yes, THIRDS is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, I 1, R 1, D 2, S 1.

Is it a valid word?

THIRDS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

one of three equal parts of a divisible whole
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)
following the second position in an ordering or series
the musical interval between one note and another three notes away from it
the third from the lowest forward ratio gear in the gear box of a motor vehicle
the base that must be touched third by a base runner in baseball

Also: one-third, third base, third gear.