A three-alarm fire broke out at a furniture store warehouse in Dorchester just after midnight Monday morning, the Boston Fire Department said. Because of the building’s size and the fire’s location — in the middle of the Logan Furniture warehouse — three alarms were ordered, according to a posting on the Fire Department’s Twitter account. The one-story warehouse at 50 Freeport St. has a showroom in the front and warehouse in the back. No one was injured. The cause was under investigation.