WASHINGTON — The Supreme Court ruled Thursday for an Arizona church in a dispute over a town’s sign law in a decision that three justices said could threaten municipal sign regulations across the country. The court unanimously agreed to strike down a law in Gilbert, Arizona, that set tougher rules for signs that direct people to Sunday church services than for signs for political candidates and real estate agents. But the justices divided over why the law violated the rights of the Good News Community Church. The church complained that the law forced the church to put up smaller signs than those for political candidates, real estate agents and others.