United Airlines flight 2857, from George Bush Intercontinental Airport (IAH), Houston, Texas to Fort Lauderdale-Hollywood International Airport (FLL), Fort Lauderdale, Florida, encountered unanticipated turbulence during descent. Two flight attendants (FAs) sustained serious injuries, one sustained a minor injury, and there were no passenger injuries. According to the flight crew, the captain, who was the pilot flying, reported that prior to departure from Houston, he conducted a turbulence mitigation briefing with the cabin crew emphasizing expected callouts and seating times. The flight was delayed approximately two hours due to thunderstorms and ramp closures at IAH. Departure and cruise were uneventful. During descent through approximately 35,000 feet (FL350), the seat belt sign was turned on. The FO, acting as pilot monitoring, made the public address (PA) to passengers and instructed the cabin crew to “prepare the cabin for landing” while descending through about 31,000 feet. According to both pilots, no turbulence was reported by air traffic control, and onboard weather radar and turbulence detection systems indicated no significant activity. The flight was operating in and out of instrument meteorological conditions (IMC) at the time. While descending through approximately FL250, the captain observed green and yellow radar returns rapidly developing directly ahead of the airplane. Before the flight crew could request for deviations, the airplane encountered a sudden, brief episode of moderate turbulence lasting approximately 3 to 5 seconds. The autopilot remained engaged, and the airplane remained in positive control throughout. The FO immediately made a PA announcement instructing the cabin crew to be seated. Following the turbulence encounter, the purser contacted the flight deck to report that three FAs had been injured. FA2, located in the aft galley, was thrown upward and landed on her left foot, sustaining a serious foot injury. FA3, while securing the aft lavatory, fell to the lavatory floor during the turbulence, twisting her left ankle and sustaining a serious injury. FA4, positioned in the forward galley, was thrown into the 1L door, later diagnosed with sprains and contusions. Two medical professionals on board assisted and examined the injured FAs during the remainder of the flight. The flight crew coordinated with dispatch and Fort Lauderdale operations to have emergency medical personnel meet the airplane upon arrival and provide medical attention to the injured FAs. The three injured FAs were transported to a local hospital. There were no other injuries, and the airplane was not damaged.