If you watch the new Stampy Long Head video where he does the races, he does the gravity falls one... There were no Dipper or Mabel racers. This has lowered my hopes for Dipper and Mabel in 3.0.
Or maybe the creators have edited the version they were playing so only confirmed characters race. Because so far we've only seen playable CPU racers when u think about it. Hopefully they just did that!