Kings are the symbols representing the 4 card suits: Diamond, Heart, Spade, and Club.
You must match either 3 of the same king, 1 wild and 2 of the same king, or 2 wilds and 1 king. Matching 3 wilds does not work.
Unfortunately, this is purely luck based - just keep trying on a daily basis! The trophy usually unlocks prior to the the first reel stopping, so you will know instantly whether or not today is your lucky day..