Step 1: Understanding the idiom.
"Once in a blue moon" is an idiomatic expression used to describe an event that occurs very infrequently.
Step 2: Explanation of "blue moon."
A "blue moon" refers to the rare occurrence of a second full moon in a calendar month, which happens infrequently.
Step 3: Conclusion.
Thus, "once in a blue moon" means something that happens rarely, making option (3) correct.