If I play Falling Star to hit an opponent's character at a battlefield, and Teemo reveals from hidden as a reaction, can I use the second 3 damage on Teemo?

Ruling: You can target Teemo with the second 3 damage only if the opponent reveals Teemo in reaction to Falling Star being played. If they wait until after Falling Star resolves and you've already declared targets for the reflexive triggers, you cannot change targets to include Teemo.

Sequence:

• Play Falling Star (no targets declared yet)
• Opponent has opportunity to react by revealing Teemo
• If Teemo revealed at this point: You can target both units when Falling Star resolves
• If Teemo not revealed: Falling Star resolves and creates reflexive triggers
• You declare targets for the reflexive triggers
• If Teemo reveals after targets are declared: You cannot change targets to include Teemo

Nuances:

• The timing of when Teemo is revealed from hidden determines whether it can be targeted
• Falling Star doesn't target units when played, only when it resolves and creates reflexive triggers