Apparently, 3ds to flt conversion (3ds Max to MultiGen Open Flight) should be doable using Blender. Unfortunately, we haven't found any relevant information to verify this and no other alternative capable of converting 3ds to flt files is known to us at this time.
Convert 3ds Max (3D Studio MAX) graphics to MultiGen Open FLighT file.
We found 3 software records in the database related to .3ds to .flt file format conversion.
Microsoft Windows software - convert 3ds to flt on Windows
A very popular free program used for work with 3D graphics
Convert from 3ds file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned 3ds to flt. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert 3ds to flt on OS X
The Mac version of popular free 3D graphics software
Linux/Unix software - convert 3ds to flt on Linux
Easy-to-use 3D modeling utility