The Clinton Administration says Fidel Castro's weekend threat to loose another Mariel-style boatlift on the U.S. is hollow. The U.S. Coast Guard may not be so convinced: Guard vessels sighted three more boats leaving Cuba today after picking up 230 frightened Cuban refugees over the weekend. The recent exodus was spurred by unrest in Havana Friday. TIME Miami bureau chief Cathy Booth says Castro, frustrated by tension during one of the Cuban economy's worst months, may let the malcontents go. In Miami, she adds, Cuban exiles went on the radio to urge their island brethren to stay put. "They don't want another Mariel," Booth says of the 1980 incident in which 125,000 Cubans fled to the U.S. "It's a safety valve -- he lets those who are angry and frustrated get out. It will allow Fidel to rule for another 10 to 20 years."