three plastic animals rule n. the legal determination that secular symbols in a government-sponsored year-end seasonal display (such as a Nativity scene) can mark it as not endorsing a specific religion or religion in general. Editorial Note: Two relevant Supreme Court cases are Lynch v. Donnelly (1984) and County of Allegheny v. ACLU (1989). Generally, such cases are related to maintaining the separation of Church and State. (source: Double-Tongued Dictionary)