All the rules to answer your questions are stated in the preview:We definitely need to see the rules on Legendary Actions.
It seems to me that it's not saying it gets a legendary action every time someone else has taken an action, but rather that is the timing trigger, like the shield spell has a reaction of when someone else attacks you or targets you with magic missile.
If it got extra actions for every opponent, the more foes, the faster it would get
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Now I wonder if the Legendary Actions are like Bonus and Reaction. You only get one of each type of action in a round. Of course that wording of "can take 3 legendary actions" might mean it can take 3 legendary actions a round, or it might mean you still only get one of those, but you have 3 different things you can chose to do when you get that legendary action. (In that case, attack, move, or chomp.) Of course, chomp taking two actions seems to me that it has to be able to have multiple legendary actions
* It can take 3 legendary actions, chosen from the list below (but Chomp counts as 2 actions);
* It regains spent legendary actions at the start of its turn;
* It can use only one legendary action at a time, and only at the end of another creature's turn.
In other words, it can use a legendary action at the end of any other creature's turn, until it has used 3, at which point it can't use any more until the start of its turn when its pool of legendary actions refreshes.
It's another attempt at handling the action economy issues for solo monsters after the various attempts in 4e. It's most like the 4e beholder (which can attack as a free action at the start of each enemy's turn if within 5 squares).