This mantra is constantly repeated, but the statistics/articles I have read prove that statement is pretty much a myth. I don't have any hard metrics on that assumption, but after some quck research, virtually every article written (usually around a local conference or conference tournament) dispells that myth. People cite Nova over G'Town in 85 or OU failing to beat KU in the '88 final, but those cases of a team failing to beat a team for a 3rd time in a season seem to be the exception rather than the norm.
This idea of "beating a team 3 times in a season" persists to give sports fans (already a pessimistic bunch) another reason to be pessimistic. And it has happened to OU several times recently (last year to Baylor and in Blake Griffin's senior season to osu) so that is clearly on our mind.
But in most cases, if a team has demonstrated the ability to beat a team twice already and is a better team, then the probability is high that they will beat them again.