Am 10.07.2020 um 23:00 schrieb bob prohaska:

The illustration that accompanied the question is of a triple,
which I've never seen anywhere in the wild and was probably
chosen for its attention getting value. It's on page 56, alas
no weblink is offered.

As a Grad student, I once had the chance to try out a triplet on the
rear seat. While I am completely unable to ride stoker on a tandem, I
was amazed to find that my stoker-panic completely disappeared on the
triplet. Probably, the mass of two other adults ensures that the
triplet wobbles more like a motorbike rather than like a tandem.