There is only one file type that uses the .MK3D extension:
Matroska 3D Video File
Stereoscopic 3D video file created in the Matroska 3D (MK3D) format, which is used for storing 3D content; uses the standard .MKV container but includes a StereoMode field for the specific format of the enclosed 3D video (e.g., side by side (left eye is first), top-bottom (right eye is first), etc.).
Updated: November 4, 2011