Is third a word in the Scrabble Dictionary

Yes, third is playable

...and is worth 8 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Third" Definition

noun

Baseball Third base.
Merchandise whose quality is below the standard set for seconds.
Music A tone separated by three degrees from a given tone, especially the third tone of a scale.
Music An interval of three degrees in a diatonic scale.
One of three equal parts.

adjective

Being next after the second in place or time .
Ranking next after the second of a grade or degree in authority or precedence .
Being the forward speed or gear next higher than second especially in a motor vehicle.
Being one of three equal parts into which something is divisible .
Being the last in each group of three in a series .