I think the most important thing was that you responded to all three students, and made it clear that they were doing something damaging to the community as a whole. (It's not just the person the harsh words in the notebook were nominally about that can be hurt by them--it's everyone who reads or writes them, or is affected by echoes of diminishing trust and friendliness such things can cause.) It's important that you responded promptly, and made it clear that this really matters.

I don't know how important it is that you assessed the relative sinfulness accurately. If a community were to decide something like, "violence is worse than fraud, so violent crime should be punished more severely," that's a concept of "worse" that points to consequences. You seem to be saying, "X is worse than Y, and you should not do either of them. Both are hurtful to other individuals and the community we value. The only thing stopping you from doing either X or Y is your conscience." In that case, I don't see the value in dwelling on comparisons that can look like excuses/permission for the lesser offense.

That said, I'm not comfortable with the idea of teaching that the second student committed a worse offense than the other two. Whistleblowing is worse than doing something wrong in the first place? If you see one member of the community writing something vicious about another community member, the virtuous thing to do is to conceal the nastiness? Can that possibly be what you meant?