An airplane encountered turbulence during a cruise flight at flight level 360 over the Atlantic Ocean, approximately 75 miles north of San Juan, Puerto Rico. One flight attendant sustained a serious injury, and one flight attendant and one passenger sustained minor injuries. The flight continued to its destination and landed without further incident. National Weather Service Weather Surveillance Radar indicated a Convective Induced Turbulence (CIT) encounter, likely associated with cumulonimbus activity, and possibly above and downwind of the cumulonimbus cloud tops.An airplane encountered turbulence during a cruise flight at flight level 360 over the Atlantic Ocean, approximately 75 miles north of San Juan, Puerto Rico. One flight attendant sustained a serious injury, and one flight attendant and one passenger sustained minor injuries. The flight continued to its destination and landed without further incident. National Weather Service Weather Surveillance Radar indicated a Convective Induced Turbulence (CIT) encounter, likely associated with cumulonimbus activity, and possibly above and downwind of the cumulonimbus cloud tops.