I want to ask about this burgle effect. "Burgle: The first player shuffles his deck and reveals the top card. He may discard 3 cards from his hand that match the revealed card's type." Means that I just discard only one card that matches revealed card? If it was necessary to discard 3 cards, the text would have used the word must.

And what about with found treasure cards. These can be used only in the following scenario, right?

All 3 cards have to match the revealed card's type. It says 'may' because you can choose to not discard any cards at all (and auto-fail the burgle attempt).

The treasure cards that you find are put into your hand, so you can use them in the Lonely Mountain scenario as you find them,

What about text on the quest card 3B "If the players defeat this stage, they have won the game and discovered any treasure successfully burgled from The Lonely Mountain." ?

If the treasure is 'discovered,' that means you are allowed to include it in your deck at the beginning of the game (so, you can use them in The Battle of the Five Armies). But because you put the card into your hand after you burgle it, you can also use it in The Lonely Mountain scenario.

ok, thx GrandSpleen. And I have another question. My revealed card for burgle attempt returns on top of my deck?

Yeah it just stays there.

Here's something to help with burgles, though: you can DRAW the revealed card, then discard it if you want. So, for example, you shuffle your deck and reveal a Burning Brand (attachment, lore, 2 cost) You now have an action window and may draw the Burning Brand/take it into your hand by using a card such as Daeron's Runes or King Under the Mountain. Burning Brand still counts as the 'revealed card' for the sake of the burgle, so now you have in your hand at least 1 card that matches type, cost, and sphere (I asked this question and got the official reply awhile back, in the thread here).