measure theory - Why is 3 a bad constant in the Vitali covering lemma?

Hi,

recently I had to do with the Hardy-Littlewood maximal function and we used there the Vitali covering lemma with constant 5. Then, given an advice, I proved it with constant k>3. But I cannot find an counterexapmle why 3 is not enough (it is enough in the finite version of the lemma).
Has anyone seen such an example?