the a<x<b syntax is permissible, and does work like @mikekaganski exemplified. However, in most cases it does not work as you may expect, and it only does because Calc does not enforce strict type. Most cases where it is used, it is done by mistake, and if it works as intended it is by sheer luck.
In the common mathemathical usage, the meaning of this type of “ternary operation” will be obvious from context. In a cell formula, allowing the use of combinations of < , = and > as ternary operator sets would introduce ambiguity of whether the relational operators are intended as two binary operators or one ternary operator set.
If it were introduced, it would require stricter type handling in Calc formulas, and this would most likely do more damage than good.