Escalating its aggressive push for a quieter spring break, Miami Beach declared a three-night curfew beginning on Friday, citing the large crowds it expects over what has usually been the peak weekend of the season.The curfew will run from midnight through 6 a.m. each night until Monday, Alina T. Hudak, the city manager, announced on Friday morning. It will apply only to South Beach, the part of the city most popular with tourists and revelers.“We did not make this decision lightly, but it should not come as a surprise,” Ms. Hudak said in the announcement. “We have been very clear…