"Just the place for a Snark!" the Bellman cried,
As he landed his crew with care;
Supporting each man on the top of the tide
By a finger entwined in his hair.
"Just the place for a Snark! I have said it twice:
That alone should encourage the crew.
Just the place for a Snark! I have said it thrice:
What I tell you three times is true."
http://home.earthlink.net/~lfdean/carroll/snark/fit1.html
Contrast
SinceWhenDoesSayingSomethingMakeItSo.
Carroll may have intended this line as comic contrast to the long-established Law of Witnesses, namely that an important fact must be established by two or three independent witnesses, not by one witness three times.