Pixar's Cars series has three main theatrical films through 2017.
The films are Cars, released in 2006; Cars 2, released in 2011; and Cars 3, released in 2017. Together they follow Lightning McQueen's development from an impatient rookie to an experienced racer and mentor.
The franchise also includes the 2006 short Mater and the Ghostlight, the Cars Toons short-film series, and the 2022 Disney+ series Cars on the Road. Those projects expand the fictional world but are not additional main theatrical Cars features.
The number three therefore refers specifically to Pixar's feature-film trilogy as of 2017. Counting shorts, television episodes, or spin-off films such as Planes would produce a different total.