Yep, as soon as you mark one K/Q/J villager, mark all three. So you only need one card to do it, as long as you can match the pattern.
Just played with my wife and 5 year old son and we had a blast! I also have one question about the “royal” villagers… If you have the pattern for, say, the K/Q/J of hearts, can you add their house when you get a face card of any suit? Or does it need to be a face card from the matching suit (hearts in my example)?