Super-late to the game, but 3331 is "sanbon tejime." Tejime is a type of clapping the Japanese do during or near the end of a ceremony as a sort of...best wishes send-off? Someone says, "Ote wo haishaku" ("Give me your [clapping] hand"), and then everyone does the tejime.
You can just clap once, or multiple times. "Sanbon tejime" means you clap three times, clap three times, clap three times, and then once. => 3331. So, it could've been done during weddings, which the song seems to be about, according to the PV.
Not discounting the "samishii" theory, which is perfectly valid, too, but the tejime interpretation is definitely one layer since "ote wo haishaku" is in the lyrics.
3331
You can just clap once, or multiple times. "Sanbon tejime" means you clap three times, clap three times, clap three times, and then once. => 3331. So, it could've been done during weddings, which the song seems to be about, according to the PV.
Not discounting the "samishii" theory, which is perfectly valid, too, but the tejime interpretation is definitely one layer since "ote wo haishaku" is in the lyrics.