Blender can both create and export AC3D geometry files (.ac) and thus is the obvious and most likely also only option for this particular conversion. You may need a special AC3D plugin installed in the program first though.
Convert Blender 3D data file to AC3D geometry data.
We found 3 software records in the database related to .blend to .ac file format conversion.
Microsoft Windows software - convert blend to ac on Windows
A very popular free program used for work with 3D graphics
Convert from blend 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 blend to ac. 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 blend to ac on OS X
The Mac version of popular free 3D graphics software
Linux/Unix software - convert blend to ac on Linux
Easy-to-use 3D modeling utility