The three mystery Fanta flavors for the "What the Fanta" campaign are as follows:

1. Blackberry: According to a review, the 2023 version of the "What the Fanta" mystery flavor is speculated to be blackberry, as it has a dark purple color and a berry-like taste.
2. Orange: Another speculation is that the mystery flavor could be a unique orange or orange-flavored product, possibly with artificial sweetener aftertaste resembling fruity, chalky candy.
3. Berry: The Fanta Zero Sugar Halloween edition includes a "Berry" flavor, which could be one of the mystery flavors.

These are the flavors that have been speculated or officially revealed as part of the "What the Fanta" mystery flavor campaign.