RIYADH - A mortar shell fired by Yemen's Houthi militia across the border into Saudi Arabia killed three of the kingdom's army officers on Friday, a Defense Ministry statement carried by the Saudi state news agency SPA said on Saturday.The statement said more than 500 Houthi fighters have been killed in clashes on the border since the conflict began on March 26, but did not say how it came by that figure. Three Saudi Arabian border guards were killed last week.