I was thinking about this the other day while listening to BDC - I think they pull off 3 members really well and the choreo in Moon Rider looks really good with 3 members (granted, they do have back up dancers too).
Personally I'd like to see more 3 member groups because right now they're really rare. Like the other commenters said, I assume it's due to the group having to perform as a duo if there are schedule conflicts or health problems. Still, it's unique and fun!