The nonprofit board that manages UM BioPark, at the edge of the University of Maryland's downtown Baltimore campus, announced yesterday that it has reached agreement with the park's developer, Wexford Science+Technology, to develop a third building in the biopark. Groundbreaking for 180,000- square-foot Building Three is planned for March 20, with construction completed by mid-2009. The BioPark's second building is nearing completion and is leasing space, with an expected formal opening early next year.