Shipping executives and union leaders for dockworkers at 29 West Coast ports ended a third straight day of contract talks late on Thursday without a settlement, despite arm-twisting from the U.S. labor secretary, but planned to meet again on Friday. Labor Secretary Tom Perez joined the talks in San Francisco on Tuesday at the behest of President Barack Obama, who has come under growing political pressure to intervene in a dispute that has rippled through the trans-Pacific commercial supply chain and could cost the U.S. economy billions of dollars.