In this subchapter, “unit of election” means:
(1) a municipality, including a principal municipality;
(2) an unincorporated area designated by a commissioners court under § 451.657 as a discrete unit for the purposes of a confirmation election; or
(3) an emergency services district operating under Chapter 775, Health and Safety Code, that:
(A) borders Lake Travis; and
(B) is located in or borders two municipalities with a population of more than 45,000.