Thanks to an alternate class feature, I've got a Sorcerer who knows one less spell per level of spell, in exchange for the ability to get domain slots like a Cleric. A prestige class I've got my eye on has "Ability to cast 3rd level spells" as a prerequisite and, well... I've only got one. The domain spell, and the slot that's assigned to it.
If this was something a player was coming to me with, I'd totally let the player have it, but the actual difference between singular and plural here is buggin' me, and while I think I'm just not gonna take the prestige class (a new DM that I don't want to bog down with endless questions about all the rules I'm using), I also want to know for future reference.