The aircraft was on a scheduled passenger flight from Birmingham (EGBB) to Malta (LMML). During descent, the Cabin Services Supervisor (CSS) was advised by the Commander that turbulence may be encountered while deviating around weather. The Commander also switched on the fasten seat belt sign and passengers were seated. Sudden turbulence was encountered while descending through Flight Level (FL)180. Three Cabin Crew Members (CCM) were at the rear of the aircraft at the time, securing the rear galley and toilets. All three fell to the floor and CCM No. 2 sustained serious leg injuries. No other injuries to the passengers or crew on board were reported to the Investigation.