If you are looking for a way to extract subtitles (such as .srt files) from Matroska movies (including .mk3d), you have to use tools like MKVtoolnix in order to get them. Should be easy enough as Matroska is basically a container, archive that contains other files, including audio track, video track and subtitles.
Convert Matroska MKV 3D stereoscopic video file to SubRip video subtitle format.
We have found 3 software records in our database eligible for .mk3d to .srt file format conversion.
Microsoft Windows software - convert mk3d to srt on Windows
A program for working with Matroska MKV files
Convert from mk3d 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 mk3d to srt. 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 mk3d to srt on OS X
A Mac version of the video editing suite for Matroska
Linux/Unix software - convert mk3d to srt on Linux
A set of utilities for handling Matroska files in version for Linux