A Roslindale trucker who allegedly kept right on driving after a large piece of metal debris flew off his tractor-trailer on the Massachusetts Turnpike, injuring three people, is now facing charges.Anthony Donaldson, 48, was charged with leaving the scene of an accident and a left-lane violation after the large, unidentified object police said was “improperly secured” struck a 2010 Subaru Legacy yesterday about 6:40 a.m.The cars were traveling eastbound in Millbury at the time.