Is het in this case associated with derde or schaap?
The het is associated with the noun, as schaap is a neuter noun, with adjectives (in this case derde) in Dutch you use the definite article that belongs to the noun. More on adjectives etc here: https://www.duolingo.com/comment/3888221 :)
Graag gedaan! :D
Oh right oops! Bedankt!