At the time the ability resolves, you'll get to play the card if you declared three different creatures as attackers at any point in the turn. A creature declared as an attacker in two different attack phases counts only once. A creature that entered the battlefield attacking (such as a token created by Militia's Pride) doesn't count because you never attacked with it.
"Hideaway N" means "When this permanent enters the battlefield, look at the top N cards of your library. Exile one of them face down and put the rest on the bottom of your library in a random order. The exiled card gains 'The player who controls the permanent that exiled this card may look at this card in the exile zone.'"
Any player who has controlled a permanent with a hideaway ability since a card was exiled with it may look at that card.
Previously, permanents with hideaway entered the battlefield tapped. This ability has been removed from the definition of hideaway. Older cards have received errata to have an additional paragraph that reads "[This permanent] enters the battlefield tapped," and they now have hideaway 4.
Hideaway now causes you to put the rest of the cards on the bottom of your library in a random order instead of any order.