I can confirm that the Wadi Murabba'at Minor Prophets scroll (MurXII - get transcriptions from here
) does indeed include Habakkuk chapters 1-3. However, the Wadi Murabba'at scroll is dated to be around the 100 CE mark, so three centuries removed from 200BCE.
There are two other Dead Sea Scrolls containing text from the book of Habakkuk - 4QXIIg and the Greek Minor prophets scroll from Nahal Hever/8HevXIIgr (again, see here
Unfortunately, the Hebrew manuscript 4QXIIg is quite fragmentary when it comes to Habakkuk, so it actually only contains text from chapter 2:4. How much exactly of Habakkuk it had is open to debate.
The Greek manuscript Nahal Hever/8HevXIIgr on the other hand does have text from all three chapters of Habakkuk, and this is dated to have been written between 50 BCE - 50CE. The fact that it's in Greek is quite significant, in that it shows all three chapters of Habakkuk together in a translation from the Hebrew, and not separated.
Therefore, the Dead Sea Scrolls really don't help in deducing whether Habakkuk chapter 3 was a later addition to Chapters 1-2 or not.
It will be said however that there really is no actual proof that Habakkuk didn't write all three chapters, or that he himself didn't add the chapter to his own writings at a later date; in the same way there's no evidence that the book of Isaiah was written by more than one person, or that the Torah was written over several centuries in order for scholars to develop their "documentary hypothesis" of how it's written.
It's all speculation, and those that would pronounce the above as "facts" would be far from telling the truth.