RICHMOND, VA — After initially resisting a takeover bid by Alcoa Inc., Reynolds Metals Co. agreed to be acquired by the world’s biggest aluminum company in a deal valued at approximately $5 billion. Reynolds is ranked third in the global aluminum industry. The merger, if completed, would accelerate a consolidation trend in the aluminum business. A pending three-way deal involving Alcan Aluminum Ltd. of Canada, Pechiney SA of France and Alusuisse Lonza Group of Switzerland would result in an aluminum company bigger than the Alcoa-Reynolds combination.