Pixar's original Cars introduced the Radiator Springs character Flo before Cars 3.
Flo is the owner of Flo's V8 Cafe in Radiator Springs and one of the town's established residents. She is a turquoise 1950s-era show car whose name and business reference the automotive culture surrounding the fictional Route 66 community. Her cafe is a gathering place for the town's vehicles.
The character appears in the first Cars film, released in 2006, and returns in Cars 2 and Cars 3. Like the other residents, Flo helps show that Radiator Springs is a community with its own history rather than simply a temporary stop for Lightning McQueen.
This question distinguishes the character's introduction from later appearances. Cars 2 moves much of its action overseas but still includes Radiator Springs scenes, while Cars 3 returns more strongly to the town and its residents. Flo is also sometimes confused with Lizzie, the elderly Model T at Radiator Springs' souvenir shop, but they are separate characters with different businesses and designs.