The DMG says that brewing a potion takes 1 day. What if my priest prepares 3 slots with the same spell and has 3 times the ingredients? Can he then make 3 potions?
Does anyone have an idea about creating batch or multiple haling potions in 1 day?
Can a creature with the feat Brew Potion brew multiple potions in a day?
No. The Benefit of the feat Brew Potion (PH 89) reads, in part,
Emphasis mine. Thus no matter how many times the creature could have brewed the potion, the creature must take 1 day to brew 1 potion.
However, potions are largely considered the most inefficient of magic items. The game would likely be unharmed were the DM to house rule away that restriction.
Is there a way for a creature to brew multiple potions in a day?
Yes. Although Pathfinder works around this naturally, in Dungeons and Dragons 3.5 such an ability is limited to prestige classes.
Conveniently, the prerequisites allow a full caster to take both to their ends. Unfortunately, the abilities that allow brewing multiple potions in a day are worded to prevent them from combining. A DM who has a PC so committed to the fine art of potioneering that the PC'd take both classes would be wise to house rule away this restriction and let the abilities combine.
No - Unless you also have 3 Clerics, it'll take 3 days
Magic Items creation rules state that: (emphasis mine)
So no matter how many materials and spells you have, a single crafter can craft a single item per day (or work on a single item for an item requiring more than one day).
Also, consider that thematically, brewing a potion is much more than merely following a chemical recipe - it is the act of trapping magic in a bottle. Not only must the brewer spend magical effort (exhausting a spell as if it was cast), but he also loses some of his own "essence" or "life force" (the XP creation cost) - this doesn't sound like something you should be able to industrialize.
You may want to use that as a houserule to avoid having to spend 4 days every time your cleric wants to prepare 4 cure-light-wounds potions...