When you search for subtitles, you will encounter the term .srt (SubRip Subtitle file). This is the standard format supported by almost all media players (VLC, MPC-HC, PotPlayer) and smart TVs.
Here are the most reliable sources to download subtitles for 3 Idiots: 3 Idiots Subtitle English Download
If you downloaded your movie from a torrent site, specifically a YIFY/YTS release, this is your best bet. When you search for subtitles, you will encounter the term
Let’s assume you have a legal digital copy of 3 Idiots (purchased on YouTube, Amazon Prime, or a Blu-ray rip). Open the movie in VLC
Subtitle → Add Subtitle File → Select your SRT.