Seventeen Pixar feature films had been released before Cars 3.
Cars 3 was Pixar's eighteenth feature film. The studio began its feature-film run with Toy Story in 1995 and released films such as A Bug's Life, Monsters, Inc., Finding Nemo, The Incredibles, and WALL-E before returning to the Cars series.
The count includes Pixar's theatrical feature films released before Cars 3, not short films, television specials, or films from other studios. Cars 3 itself was released in 2017, after Finding Dory and before Coco in Pixar's feature-film chronology.
This numbering is separate from the franchise numbering. Cars 3 is the third Cars feature, but it is the eighteenth Pixar feature overall. That distinction makes the answer different from asking which installment of the Cars story the film represents.