The Pennsylvania Turnpike will replace three bridges and lower a road in Plum and Monroeville over the next four years to prepare for a 7.5-mile, $217 million widening project. Officials will hold an open house Thursday to display plans and answer questions about the work, which is part of the turnpike’s effort to rehabilitate and expand the length of the cross-state toll road. Some parts of the highway haven’t been upgraded since it opened in the 1940s.