Dungeon of Utter Uncharitibility
 

I forget the name of the Dungeon but it allows you to have as many cards as you want in your hand. It also states that upon leaving the Dungeon you have to return your hand size to its legal limit.

So, we start the game with this Dungeon in play. I am the first to go in a 3 player game. I take my turn normally and before ending my turn, I play a Portal that discards the current Dungeon and puts us in a new one.

The rules state: When a Dungeon card is discarded, follow its instructions about
reversing effects, discarding extra cards, and so on.

As soon as we leave the first Dungeon, do the other 2 players have to go down to 5 cards? (I was already fine after making my plays) Or is it that since they are allowed to have any number of cards on someone else's turn they can keep the extras until the end of their turn?

Re: Dungeon of Too Many Cards
 

The players must have a 5 cards hand max (6 cards max if dwarf) at the end of their respective turns.

So, they could start their turn with many more cards than 5, and then go through the different phases in a turn, but by the end of it they will have to do the appropriate charity/discards in order to have the maximum allowable amount of cards.

Re: Dungeon of Too Many Cards
 

The Dungeon of Utter Uncharitibility dictates that as soon as you leave the dungeon, everyone goes down to 5 cards (6 if Dwarf) and all extra cards are discarded; no one receives charity.

Re: Dungeon of Too Many Cards
 

That is basically correct. The wording is pretty clear about what happens when the dungeon is left by a player or, for that matter all the players, as when everyone leaves, the instructions apply to everyone. If there's any question about why it's worded the way it is, that would be because of Narrow Passage, where one player leaves at a time, meaning only as a player goes through the Narrow Passage, do they have to discard down to 5 (or 6 for Dwarves). However, when everyone leaves at once, they must all discard down to the appropriate number of cards.