Clogged ports along the U.S. West Coast were closed to cargo freighters for the third time in a week on Friday night, and shippers vowed to keep a partial shutdown in effect through Monday barring a settlement in labor talks with the dockworkers’ union. The latest halt in loading and unloading of container ships at the 29 ports came as contract talks between union negotiators and management’s bargaining agent, the Pacific Maritime Association, appeared to reach a critical juncture.