Addersgall regenerates out of combat. So, Sage starts a fight with 3 stacks.
SCH can use aetherflow right at the start of an encounter. So, Scholar starts a fight with 3 stacks.
AST is a little different - nothing quite like addersgall/aetherflow/lilies - it's cooldown based, but this also means Astro starts a fight with full charges and CDs ready to go.
And then we come to WHM. Lilies do not generate out of combat - so white mage will start a fight with 0 of its healing resource, and won't get 3 until 90 seconds in.
Why is WHM the odd one out here?