I heard of this novel but never read it. It is based on the Bellman's line "What I tell you three times is true", in Lewis Carroll's "The Hunting of the Snark". The entire plot is about convincing aliens that only information obtained three times in a row can be trusted, and in that way saving Earth from an invasion, as far as I have understood.
"Chaos, Co-Ordinated", published in Astounding Science Fiction (1946), matches.
There's also a short story called Chaos, Co-Ordinated published in Astounding Science Fiction (1946) in which the humans manage to feed The Hunting of the Snark into the alien supercomputer as a field report and thus save the earth, largely because the computer now runs on the Rule of the Bellman and wipes its own memory of everything that it has heard only once.
You can read it in the Internet Archive copy of the magazine.
I found it by searching for novel aliens "tell you three times".