Let me make a few assumptions first, since there isn't enough information in the first post. I will assume the healer would be able to cure any target at any time during the fight. I'm acting on the assumption that being labeled as a cleric does not define the character as a western armored cleric, but a character akin to the white mage in the picture. The mystic will cast buffs that will increase the attack and defense values of everyone involved, including himself. He will not be protected against combat, provided he does not have any buffs. Finally, I assume the knight is intended to deal high amounts of damage in order to draw my attention to himself. He doesn't have any actual abilities to force me to fight him, but he is a dangerous combatant to ignore. I imagine he is not a soft target, and will not go down easily.
If I have initiative, I will destroy the mystic. If he successfully buffs the defenses of other characters, my damage will be nothing but chips, including the healer. From there, the cleric must be defeated, but now that the mystic is out of the way, I'll also apply any crowd control effects I have to the knight, to protect myself from him long enough to finish the cleric. Once the cleric is defeated, I will then fight the knight.