FM3 (Lotus 1-2-3 Spreadsheet Format File)
The FM3 Settings file format
FM3 files are created by older versions of the spreadsheet software created by Lotus 1-2-3. Files with the FM3 extension are no proper spreadsheet files. Instead, they contain formatting information for spreadsheets saved with the WK3 extension. This separation between formatting and actual spreadsheet files containing the data as it was handed by Lotus 1-2-3 is not used nowadays anymore. In modern spreadsheet files, the formatting information is saved in the spreadsheet as well.
Technical details of FM3 files
In contrast to the WK3 extension that is used by Lotus 1-2-3 to store the data inserted into a spreadsheet, FM3 files only contain display formatting information for a spreadsheet. These information include the formatting of cell borders and shading, fonts, as well as other elements. Some older versions of Microsoft Excel are still able to open FM3 files.