https://live.staticflickr.com/65535/48339353806_7488fa57c3_b.jpg

Linda Kaye Henning (Betty Jo), Jeannine Riley (Billie Jo), and Pat Woodell (Bobbie Jo) - Seasons 1-2

https://m.media-amazon.com/images/M/MV5BMTcwNzU3NjUxNV5BMl5BanBnXkFtZTgwMzExMTU3MTE@._V1_.jpg

Linda Kaye Henning (Betty Jo), Gunilla Hutton (Billie Jo), and Lori Saunders (Bobbie Jo) - Season 3

https://m.media-amazon.com/images/M/MV5BMTAyNTE3ODI3NDleQTJeQWpwZ15BbWU3MDY5MTA1ODI@._V1_.jpg

Linda Kaye Henning (Betty Jo), Meredith MacRae (Billie Jo), and Lori Saunders (Bobbie Jo) - Seasons 4-7

Here's another discussion (https://moviechat.org/tt0056780/Petticoat-Junction/58c724915ec57f0478ed8eba/Which-combination-of-Bradley-sisters-was-the-best) on this particular topic in greater detail.