If my opponent has less than 3 cards in his or her deck, so that I can't reorder the top 3 cards, can I still use Slowking's "Second Sight" Poke-POWER to look at the remaining cards in the deck?
Yes, you look at your opponent's remaining cards, then put them back in the deck in any order.
Source: HeartGold & SoulSilver FAQ (2010-02-11), PUI Rules Team (2010-02-11)