This implies that is possible to investigate all the three of them at the same time... but how is that this can be done? What does it mean, in very practical terms?
It's possible to investigate all three at once because they occur simultaneously, it requires precision to really experience the entirety of each sensation but it's possible with practice. In practical terms, what you're try to experience fully is how each sensation is:
a) Impermanent: Evident in the fact that each and every sensation can be, and is, seen to arise and pass of it's own accord; whatever it is, it's there and it's gone; regardless of 'time' (a human-created measurement of naturally occurring entropy[1]), it's evident that nothing in existence is truly permanent and unchanging; "be" impermanence, experience it directly in all phenomena; notice how the sense of an unchanging or permanent observer isn't excluded from this either...
b) Unsatisfactory: This is already implied by a) since nothing transitory, which means any and all phenomena, can be held onto and so can't provide satisfaction. What happens if you try to hold onto any experiences in life? Look at the way the mind pulls towards the past and compares this experience to one that's gone before, if you're constantly living in this state of comparison then you're not really here, right now; "you" are always in the past or in the future unless you're paying attention to what's going on right now.
c) No-self: But wait! This "you" who's always in the past or in the future? Where does that go when you're here right now? When you're comparing experiences or trying to predict outcomes, "you" seem to exist in the memory of the experience or in those imagined potential outcomes; "you" seem to exist as the one who's doing the thinking of those thoughts, but look closer: The "you" who thinks it's the thinker of these thoughts only exists as the thought itself. Can you find it in any of these transient, unsatisfying sensations? Is it apparent in the gap between thoughts? It's happening right in front of you.
Combining all three isn't all that difficult if you're well concentrated and able to note the arising and passing of sensations accurately. I've tried to demonstrate how they're all evident in everything you can possibly experience, it's just a matter of doing it and understanding what each of the 3C's refer to experientially. You don't need to "note" anything, they're just concepts to communicate something fundamental about this experience as it happens, they're there to be seen if you know how to look.
T
[1] Entropy used in
this sense: "The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity; Inevitable and steady deterioration of a system or society."