''"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

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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.