Having thought hard about its design I have ended up with this plan: a central teepee of rustic poles, supporting six "Cherokee Trail of Tears" bean plants; one "Butterbush" squash plant at each end of the bed, surrounded by six "Mirai Yellow" sweetcorn plants.
This is how it looks at present (not finished yet - and those lettuces have to go). The teepee of poles is made mostly from branches cut from my Philadelphus tree - more sustainable than bamboo poles imported from China! They are about seven feet tall.
At the base of each pole is one of these bean plants.
I have planted one of the squash plants under a big plastic cloche to provide protection from cold night-time temperatures. This is surrounded by six sweetcorn plants which are protected from animals and provided with support by a ring of plastic Clematis netting held in place by three short bamboo canes. I reckon that if it becomes necessary to do so I should be able to cover all this with a piece of fleece.
I will repeat this arrangement at the other end of the bed in due course. I'm hedging my bets again, by keeping a few plants in pots so that they can be brought under cover if necessary.
I know this arrangement of mine involves a bit more "hardware" than the original Three Sisters concept, but I think the Sweet Corn would probably not be able to adequately support the bean plants, and I don't trust my local foxes to leave things alone. We'll see how it goes, and maybe make some adjustments next time round if I get some success.