There are a number of word lists whose words were picked due to their beneficial properties given the use-case of possibly needing to be understood verbally over unclear connections. The NATO phonetic alphabet, and PGP word lists come to mind: https://en.wikipedia.org/wiki/PGP_word_list

I'm particularly a fan of the PGP word list (it would definitely require more than 3 words for this purpose, though) because it has built-in error detection (of transposition, insertion or deletion): Separate word lists are used for "even" and "odd" hex digits. This makes it, IMHO, fairly ideal for use over verbal channels. From the wiki: "The words were carefully chosen for their phonetic distinctiveness, using genetic algorithms to select lists of words that had optimum separations in phoneme space"

It sounds like the w3w folks did not do any such thing

EDIT: According to my napkin math, 6 PGP words should be enough to cover the 64 trillion coordinates that "what3words" covers, but with way better properties such as error detection and phonetic incongruity (and not only that, it is just over 4 times larger, which means it can achieve a resolution of 5 feet instead of 10)