I think if you put it on your player, villagers will randomly walk up to it and sing. Having the three sing together like this is a random occurrence, and I think they just pick songs to sing either from what's playing nearby or what they've heard before.

Thanks! Them singing it was so cute. I want to put it on my player to see if mine’ll sing it too!

Leave the damselfly alone. It just wants to be a part of your happiness.ever thought it might be its favourite song too?!

My 3 cutest villagers singing my favorite song under my favorite weather effect. You can't hear me yelling at the damselfly to stop ruining the moment.