Write down all of the dreams you can and look for any patterns. There won't be a dream sign that occurs in every dream or every night but there will probably be one that occurs fairly often, like 4 or 5+ nights a week. Your dreams are based on your waking life. I'm sure your waking life is consistent in some way which leads consistencies in your dreams.

Remember, a dream sign can also a consistent idea, emotion, or sensation as well. Searching for dream signs beyond just simple images will help you find some. For example, I can see just from your 3 examples, two of them contained idea of a dark dream character.