After decades of starts and stops, three air-rights projects over the Massachusetts Turnpike are gaining traction. The projects would construct structural decking over Boston’s busiest highway. The $600-million Fenway Center is the closest to construction and would include three structures over the turnpike near Kenmore Square. A second project recently rebooted its original 400-ft tower plan. The new proposal calls for a mid-rise and a 24-story apartment building that would require 22,000 sq ft of decking. Finally, two other developers reportedly are interested in reviving the $800-million Columbus Center. The 10-acre mega- project would connect the city’s Back Bay and South End neighborhoods by covering the highway.