DM3 format is a special digital image format used in microscope images processed with Gatan DigitalMicrograph software and not really a picture format supported in standard graphics editors.
Whilst some picture converters might be able to save to a variant (usually older) of the DM3 format, these are essentially RAW files with special metadata that can be only captured with the microscope device.
This means that while tiff to dm3 conversion is in theory possible, the resulting file would lack any special metadata that would be otherwise expected to be part of .dm3 file.
Updated: January 8, 2024