A 21-year-old woman was flown to the Maryland Shock dTC Trauma Center in Baltimore with head injuries last night after she was thrown from her car in a collision with a pickup truck at Route 27 and Snydersburg Road. Two passengers in the woman's car, both young men, were taken to Carroll County General Hospital for treatment of injuries they received in the 6 p.m. crash. The driver of the Ford pickup truck was taken to the same hospital. State police said the woman apparently was not wearing a seat belt. Investigators said the woman, headed west on Snydersburg Road, pulled onto Route 27 in front of the pickup. The truck struck the left front of the Nissan Sentra, which spun around at least once before it came to rest in the southbound lane, police said. The woman was found lying in the road by police. Names of the people involved were not released by police, pending notification of their families. Police said the woman had been cited recently for failure to wear a seat belt.