Nokia and Microsoft teamed up to build an iPhone killer on Friday in a desperate attempt to take on Google and Apple in the fast-growing smartphone market. In a strategic u-turn, Nokia said it would use Microsoft’s Windows Phone as the software platform for smartphones. “It’s now a three-horse race,” chief executive Stephen Elop said. Investors were not convinced by Nokia’s announcement and the company’s shares tumbled more than 11%. The deal would have a negative impact on Nokia’s margins in the short-term, analysts said. Nokia said its operating margin would be “10% or more” after the transition period.