For those of us who aren't fully versed on all the Halo lore, etc... I'll say this plainly so that you don't have to go hunting for the answer like I did.
In order to get all 6 of the Vidmaster Challenge: Brainpan skulls, *all you need* is Halo: ODST. I'm saying this so that you know you don't have to buy the Mythic pack for Halo 3. ODST has those maps on the second disc, and those maps do contain the 3 skulls from Halo 3.
So in other words, if you have ODST, you can get all 6 skulls, including this one, on the multiplayer disc (very, very easily, and get yourself a quick 175 GS in under an hour), or you can use Halo with the two Mythic packs. All of the skulls are available in both places.