PARIS, March 17 (Reuters) – U.S. tech giant Microsoft (MSFT.O) is facing an antitrust complaint filed by three European rivals in the booming cloud computing business, one the plaintiffs said on Thursday. The complaint, filed with the European Union’s competition watchdog months ago, alleges that Microsoft’s contractual and business practices make it costly and difficult for users of its cloud computing services to opt for those of a competitor, a source close to the matter said.