This was also confusing to me in the beginning. One answer is that by simply noting, you are seeing the three characteristics. As long as you note different things over time you must notice that sensations are not permanent. The fast your noting becomes the more you see that your whole sense field is changing the whole time (impermanence). Then as you start noting the arising and passing of things like your thoughts, your intentions, mental volitions, urges, reactions, you start noticing that these either seem to arise due to a clear cause (smelling bacon, intention to move to the kitchen appears), or seemingly coming out of nowhere. In both case all those things, and indeed all things in your sense field are 'just happening". (not self). Then you may start noting tensions in your body, the sudden tension associated with push-pull, or basically any time that you are not fully in tune with the fact that everything is changing and just happening by itself (suffering).
So even without looking for them directly, by simply noting arisings and passings, and becoming more and more inclusive over time the 3Cs will reveal themselves to you.
Another approach is to go at them directly (I often do this now): just get in tune with your sense field constantly changing. Then get in tune with the fact that everything just kind of happens by itself. Then get in tune with the subtle tension in your body and mind as you resist these facts. This could be called a way of looking keeping a certain (or all three) characteristics in mind.
In the beginning the notice practice is a nice crutch. Later on, or when you get to equanimity you can go more freeform.
Good luck! Happy sitting!
