Chipotle founder and ex-CEO Steve Ells is plotting a return to the fast-food scene — with a plant-based, robot-powered chain whose restaurants could operate with as few as three employees, The Post has learned. Ells, 56, has been pitching investors since at least last year on a startup that “fundamentally rethinks of labor, technology, real estate and menu” and uses “automation with a human touch,” according to draft investor materials obtained by The Post. “We proved a 3-person labor model can work,” the company said in the materials, which The Post did not publish in order to protect the anonymity…