Fire 3 and Fire 2 are completely different. Fire 2 is an AoE and Fire 3 is a single-target spell. Use Fire 3 at full mana (for 3 stacks of Astral Fire), and then whenever Firestarter procs off of Fire 1.
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Completely different spells. Fire 2 is your AoE, for use on large, weak groups. Fire 3 is used to switch to the fire part of the fire/ice cycle as it instantly gives you Astral Fire III, and when Firestarter on Fire 1 procs. Fire 1 remains your primary Fire attack spell throughout.