Based on legacy download counters from 2008–2015, these songs were the kings of 3GP sharing:
Optimal for newer feature phones with slightly larger screens. 2. Reliable Methods for Downloading Tamil Video Song 3gp Quality Download
You are not getting "HD" or even "SD" quality. You are getting functional quality. Lyrics should be readable if subtitled. Faces will be blocky during fast-motion dance sequences (like in "Vaathi Coming" or "Why This Kolaveri"). But for audio-centric viewing on a small screen (1.8 to 2.4 inches), it is perfectly acceptable. Based on legacy download counters from 2008–2015, these
If you have MP4 files on your PC and want to shrink them: You are getting functional quality
As technology advanced, the limitations of the 3GP format began to outweigh its benefits. The transition from feature phones to smartphones (post-2008 with the rise of Android and iOS) changed user expectations.