Even though I have never seen an issue with a triple axle configured rubber torsion axle trailer doesn't mean that the problem has never existed. The Airstream is a relatively light trailer per axle. If I was looking for a used triple axle silver RV trailer, I would get a mid 1988 Avion or older trailer so I would be buying the leaf spring independent axle with a heavy frame. One of our members have owned the Airstream triple and the Avion triple and he is very happy with the Avion, besides it is probably cheaper.
There are enough axle manufacturers that don't recommend the triple axle configuation with the rubber torsion axles that I wouldn't intentionally try to find a trailer with that configuation. My guess is that they have seen problems with the triple axle configuration, but they were probably on the heavier weight trailer with these type of axles. When you think of the leverage placed on the triple axles from the front axle to the rear axle when in a tight turn, I can see the problem occuring by pulling the torsion arm assembly partially out of the axle tube. Maybe it has never happened, but if that is the case, why are the manufacturers recommending against it.
I have never seen an equipment trailer with the heavier (7,000#) rubber torsion axles in the triple axle configuration. My guess is the heavier the load, the more of the problem with this setup. Also, the rubber torsion axles don't have an equalizer so if you turn in an area that has a dip while turning, your front and rear axle could be carrying most of the weight while you are applying the horizontal leverage against the axles.