Whilst I was waiting at the traffic lights at the junction of Chelsea Bridge Road, Royal Hospital Road, Lower Sloane Street and Pimlico Road, this morning, a chap went around the corner on a triplet, albeit empty on the back.
I only saw it very briefly, as the lights changed, and traffic got in the way of my view, but it looked like it was a double child back, presumably the occupants having been previously dropped off at school or similar.
Triplets are fairly rare, but I suspect child back variants make up a reasonable number of them, since transporting children around is a good use for one.