Three of the four groups below are built on one and the same rule. Which group is built differently? C3F, F6I, I9L, L11O

In each group the number gives the place of the opening letter in the alphabet, and the closing letter stands three places further on. C is at 3 and F at 6; F is at 6 and I at 9; I is at 9 and L at 12. L, however, stands at place 12, not 11, so L11O breaks the rule even though its two letters are three places apart as the others are. C3F, F6I and I9L each answer to the rule exactly; the run of opening letters C, F, I, L is regular throughout, so the position of a group in the run is no guide.