WASHINGTON — President Donald Trump on Saturday ordered a new trade investigation that could heap more tariffs on imported lumber, adding to existing duties on Canadian softwood lumber and 25% tariffs on all Canadian and Mexican goods set to take effect next week. In his third new tariff probe in a week, Trump signed a memo ordering Commerce Secretary Howard Lutnick to initiate a national security investigation into U.S. lumber imports under Section 232 of the Trade Expansion Act of 1962. Trump also used the trade law to impose tariffs on global steel and aluminum imports. The probe covers derivative