A cache of arms that included three mortars, 30 machine guns and 2,000 rounds of ammunition was uncovered in Brooklyn last night hours before it was to be turned over to anti-Castro Cubans the police reported.

Three persons were arrested.

Acting on a tip, detectives from the Brooklyn Safe and Loft Squad hid in the shadows outside a store in the Boro Park section and arrested two men as they carried two mortars to a parked car.

The detectives Edward Lehane and Tobias Stegman, moved into the M and D Sport Shop at 6422 14th Avenue. They said they had found the weapons in the rear and basement.

In addition to the mortars, machine guns and ammunition, the police said they found 12 60-millimeter mortar shells, 5 German submachine guns, 6 automatic rifles and 12 automatic pistols.

The buyer, one of the men arrested, was described by the police as a Cuban who lives in the Bronx.

The owner of the store was also arrested. Although the shop sold hunting rifles, which were on display in its small window, it is not allowed to sell automatic weapons, the police said.

The third man held by the police was described as a friend of the Cuban.

Treasury Department agents were questioning all of the suspects early today.

The police said that it was no immediately clear how the weapons were to be shipped to Florida but that they were to be sent sometime early today.

The discovery of the cache came only three weeks after indictments were dismissed against two Cubans and a Cuban-American who had been accused of firing a bazooka at the United Nations while Ernesto Che Guevara, an aide to Premier Fidel Castro, was speaking there. They said they had meant only to take attention away from Mr. Guerava.

The police said that the sporting goods store, which has been open for about a year and a half, had been under surveillance for some time.