Currently you can only stream Cars 3 with a Disney+ subscription, as with almost all animated Disney movies. We will update this page if we find more streaming options in the future.
If you don't have access to Disney+ then you can also buy or rent Cars 3 to watch online using platforms like iTunes, Google Play Movies, Amazon Prime Video, VUDU, and Microsoft Store. It'll cost you as little as $2.99 to rent the movie and $9.99 to buy the movie.