The parent company of Simmons Bedding Co., the Atlanta-based mattress maker that filed for bankruptcy three years ago, has been sold.

The new majority owner is Advent International, a private equity firm based in Boston. Advent International acquired AOT Bedding Super Holdings, parent company of the National Bedding Co., which is the majority owner of Serta and Simmons. The deal was announced Sunday and is expected to be completed by the end of the year.

In 2009, Simmons was sold to an investor group led by Los Angeles-based Ares Management and Teachers' Private Capital, the private investment arm of the Ontario Teachers' Pension Plan, for $760 million. Both will maintain minority stakes in the company.

Simmons, which moved its headquarters to Atlanta in 1975, employs about 3,000 people at 19 plants across the U.S. A spokesman for Simmons said there are no plans to move employees. Terms of the deal were not disclosed.

According to a statement from the companies, Serta and Simmons will maintain separate sales and marketing. In a statement, Advent International principal Jefferson Case said the companies have "strong momentum in an industry we believe is poised for growth."