Crews battled a house fire in Charlestown late Sunday night. Firefighters responding to a reported fire on Allston Street just before 11:30 p.m. found heavy flames coming from the 2-and-a-half-story building, according to the Boston Fire Department. Three occupants were able to get out safely and there were no reported injuries, fire officials said. The blaze left behind about $500,000 worth of damage. The cause remains under investigation.