It should be possible to export the geometry from X3D graphics models (.x3d) to a standard AC3D geometry file (.ac) with programs like Blender or similar 3D modeling tools or even some dedicated converters. This is probably used when you need geometry only, for a program that accepts .ac files only.
Convert X3D graphics model to AC3D geometry data.
We have found 3 software records in our database eligible for .x3d to .ac file format conversion.
Microsoft Windows software - convert x3d to ac on Windows
A very popular free program used for work with 3D graphics
Convert from x3d 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 x3d 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 x3d to ac on OS X
The Mac version of popular free 3D graphics software
Linux/Unix software - convert x3d to ac on Linux
Easy-to-use 3D modeling utility