Nope. The first party are WotC, the people who make the game. The second party in this instance is me - the person who buys the game. A third party, then, is anyone else.
I wrote a third-party product in 2011. It is called Outer Veil and is a brand-new third-party setting for Mongoose Traveller published by Spica Publishing.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.