Example 1
Person A chooses to sacrifice Negate.
Person B chooses to sacrifice Dodge.
Person C chooses to sacrifice Deflect.
All three lose access to that use of that skill for the remainder of the encounter - it is considered spent and may not be refreshed.
At any point during the encounter, each of the three may choose to use any of the three skills one time. So if Person A is targeted by an attack, she may choose whichever defense is applicable, but then may not use either of the others for the remainder of the encounter.
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Example 2
Person A sacrifices Break Limb
Person B sacrifices Stunning Blow
Person C sacrifices a rank II Spell Slot, choosing the spell Enigmatic Lance
All three lose access to that use of that skill for the remainder of the encounter - it is considered spent and may not be refreshed.
At any point during the encounter, Person A may choose to invoke any of the three attacks, one time. So if she chooses to activate Break Limb, she cannot use the Stunning Blow or Enigmatic Lance.
OOG: Zelda Turqman