In the short term, it will probably be a very distant third. In the long term, as the wild west gets settled and mobile computing hardware stabilizes toward longer life cycles, I think Microsoft will be more of a player in the business world. BYOD works better when the mobile device is not the primary computing interface within a business. As the mobile device becomes a primary computing device for line of business applications, the vertical integration provided by the Windows mobile platform provides serious advantages.
The competing operating systems are not driven by developer support because the underlying businesses are hardware sales and data aggregation. Microsoft is a tools company.