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Huawei unveils world’s first tri-fold phone, the Mate XT Ultimate Design
(spillsomebeans.com)
This is a most excellent place for technology news and articles.
I guess the same way you slice something into one piece. Bisect is one less than trisect, so what's one less than bisect?
Maybe the problem is that the phone is rectangular. I bet they could do it with a polygon with just two fewer sides. ;)