The grassland enclosure counts the enclosures you bought and placed. However, your zoo started out with three enclosures, and those don't count towards the achievement.
You can still place more grassland enclosures and remove them later, or you can put some of the animals you already have in there (sooner or later your enclosures will be full anyway, since there's a limit to how many animals of the same kind can be kept in a single enclosure).