Thank you, l'il brother, your opinion as a musical guy matters to me :) I believe this style is called doo-wop, or close harmony singing. And, I like how they elaborate on it, in the interview I linked to:

Our experience with our audience is that one thing that brings immediate joy to people is harmony. It’s not even a metaphor; harmony is actually what we need; to harmonize our world.

Wherever we go with our music, we have three voices melding in harmony.