Forum:Question on "Declaring Attacks"

In a recent duel, my friend had a continuous trap card on the field (Unfortunately, I can't remember the name of it at the moment) which said something along the lines of "Whenever your opponent declares an attack, he/she must choose one random card from your hand and choose whether it is a Spell, Trap of Monster card. If he/she chooses wrong, the attacking monster is destroyed." He also had three set monsters. I summoned Elemental Hero Wildedge and attacked, but we couldn't decide whether this would be considered declaring one attack against all three monsters (Causing me to have to choose one card) or declaring three separate attacks (Making me choose three). We went to his wife for an unbiased opinion but I want to know the official rules for a situation like this. Anyone know?