In that case it could've been two different users. And even if it was the same, it could still be legitimate (judging only from the titles, I'd say the first question is awesome, the second I have no universe knowledge and the third smells subjective). A case of three votes from the same user to the same user in either direction doesn't qualify as a red flag, if you ask me. Only if this happens repeatedly to the same user, I'd start asking for help.
I know how frustrating serial down votes can be, but think about how great serial up votes feel, especially for very old questions. And the latter happens more often than the former, judging from my own experience.
It happens if somebody has a look at your profile and checks out some of your posts.
Anyway, to more directly answer your question: I'm pretty certain that -- if you can detect serial up votes -- you can detect serial down votes. But I have no knowledge of the SE implementation. I would guess that there exist measures to detect massive abuse.