Not the "third worst" -- the "third most horrible". I think Breya measures "horribleness" in moral/ethical terms, unlike "badness" which can just be ordinary coarseness, obscenity, etc.
And I'm guessing that at least one of the first two was during the recent story, e.g., somewhere around here
. Nothing comes to mind for the other -- it could be something else that's happened in the Outjack system, or something from back during the Buuthandi battle or when she was still with the Toughs. I'm pretty sure there's something from back then that's horribler than anything recent, but the recent stuff would be fresher in her mind.
My hunch is that there's one thing from back during their earlier association that so burned itself into her mind that it stands out as the "most horrible" (though I don't know what it would be). Then #2 is recent and #3 is right now.