I am a druid who plans on taking advantage of wild shaping to use a form with multiple arms to wield multiple weapons such as an octopus. Multiweapon fighting is the prefect feat for this. However it requires three or more hands. So since I will have the ability to gain these arms but do not actually posses them naturally, how do I take the feat?
On Feats on Prerequisites says
Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat. A character can gain a feat at the same level at which he gains the prerequisite.
That is, first, a creature must meet a feat's prerequisite to select the feat at all. Thereafter, a creature need not always meet that feat's prerequisite to keep the feat, but the creature must meet the feat's prerequisite to realize the feat's benefit.
Further, Character Advancement on Advancing Your Character says, "A character advances in level as soon as he earns enough experience points to do so—typically, this occurs at the end of a game session, when your GM hands out that session’s experience point awards."
This means, for example, if a creature meets the prerequisite of the feat Multiweapon Fighting while using a special ability right as the creature's about to advance to a level that allows the creature to pick a feat, the creature can—assuming the GM's okay with it—select the feat Multiweapon Fighting when he advances to that level. However, the creature will only be able to make use of the feat when he can meet the feat's prerequisite.
In such a way, a druid can take the feat Multiweapon Fighting by advancing to a level in which he's allowed to take a feat if the druid advances to that level while in octopus form and if the druid's player's GM rules that an octopus's tentacles are sufficient to meet the three or more hands prerequisite of the feat Multiweapon Fighting. Also, of course, the druid must still meet the feat's Dexterity 13 prerequisite.
Talk this over with the GM first, though, as the GM will probably want a say in whether this practice is permitted at his table.
Jason Bulmahn (Paizo's Lead Designer) once said that you must have a permanent way to obtain the ability so you can meet requirements:
Generally speaking, you need to permanently have a score, feat, or ability to meet the prerequisites of a feat or prestige class. In the case of a magic item that boosts an ability score, you need to be wearing the item full time for it to be considered a permanent bonus for this purpose. It is one of the few ways you could take a feat that you later do not meet the prereqs for.
Personally, I would say that have claws for a few rounds per day does not meet the prereqs for a feat that requires a claw attack. Just like being able to cast beast shape once per day would not grant you the prereqs. Note though that this is an off the cuff opinion. I would need to investigate further.
While he admits it is just his opinion, the lack of any further developer commentary on this topic has caused his opinion to be widely accepted as rules as intended by the community. Even though some GM are encouraged to rule however they want at their tables.
Do notice that this was back in 2011, and the developers have issued many FAQs that has shifted their point of view on certain topics, like the difference between temporary and permanent ability score penalties by James Jacobs (Paizo's Creative Director). This topic has been flagged for a FAQ with 25 voters but still has no final answer from the developers, so i wouldn't be surprised if the official ruling about this changes at some time on the future.
This means that you cannot qualify for Multiweapon Fighting because your wild shape duration is not enough so your bonus is considered permanent (usually, 24 hours) and you do not have three hands on your natural form.
Prerequisites: Dex 13, three or more hands.
In my experience as GM, I really don't see many problems with allowing a character to buy something that they qualify only a during a certain period of the day, or for a limited time. This even has precedence on the rules with the Fly skill.
You cannot take this skill without a natural means of flight or gliding. Creatures can also take ranks in Fly if they possess a reliable means of flying every day (either through a spell or other magical manner, such as a druid’s wild shape ability).