Three teens were seriously injured when their car slammed into a tree following their prom — and police don’t know how long the crash went undiscovered. A Montgomery County police officer came upon the wreck on Middlebrook Road about 6:15 a.m. Saturday, investigators said. Police said it appears the driver, may have fallen asleep while driving his 1992 Buick Century. Investigators said the car struck a curb, then slammed into a tree. It took rescuers 15 minutes to free the driver and his two passengers from the mangled Buick, police said. The driver, most seriously injured, was flown by helicopter to a nearby trauma center, where he was in critical condition, police said. Investigators said it the students from Northwest and Clarksburg high schools were returning from a prom-related event. The crash happened about a mile from the Northwest High School, which held its prom Friday night.