|Star Trek: The Original Series-Season 3||R4|
Star Trek: The Original Series came out on DVD in R1 little by little on two-episode single DVD releases, beginning in August 1999 and ending in December 2001. Each release had the original trailer previews but nothing more, and these episodes were released in production order, not in the order of the original air date.
Star Trek: The Original Series – The Complete Third Season was released on DVD in R1 on 14 December 2004. This box set appears to be substantially identical to the R4 release, although the R4 release again has many more foreign language options.
Given that there is little to distinguish the two, I suggest purchasing whichever is cheapest.