Also, mention alternative tools, like FFmpeg for re-indexing, but mkvtoolnix is more user-friendly. Maybe provide an example command with FFmpeg in case they prefer it.
Finally, a note on legal aspects: Using this guide should be for content you own legally. Remind them to respect copyright laws.
Potential step-by-step guide: 1. Install mkvtoolnix. 2. Open the .mkv file in the tool. 3. Re-index. 4. Save as a new file. 5. Play the new file.
Another angle: The user might be in India, since "3 Idiots" is a popular Bollywood film. They might have torrented the movie, which sometimes comes without proper indexing. So explaining how to re-index using mkvtoolnix would be helpful.
Make sure the answer is thorough but not too technical. Assume the user might not be familiar with command-line tools, so focus on the GUI tool first, then mention the command-line option.
Check if there are common issues with "3 Idiots" .mkv files. Maybe some torrents have issues. Suggest re-indexing as a solution.