In the Twilight series, the three types of life forms mentioned are werewolves, vampires, and humans. Werewolves are individuals who can transform into wolves, vampires are immortal beings with supernatural abilities, and humans are ordinary people without any supernatural traits. These three groups play significant roles in the story, with conflicts and alliances forming between them. The presence of werewolves, vampires, and humans adds depth and complexity to the narrative, exploring themes of love, identity, and the struggle between good and evil.