here's one well-written take on the subject:
Thanissaro:
'Almost any book on Buddhism will tell you that the three characteristics—
the characteristic of inconstancy, the characteristic of stress or suffering, and the
characteristic of not-self—were one of the Buddha’s most central teachings. The
strange thing, though, is that when you look in the Pali Canon, the word for
“three characteristics,” ti-lakkhana, doesn’t appear. If you do a search on any
computerized version of the Canon and type in, say, the characteristic of
inconstancy, anicca-lakkhana, it comes up with nothing. The word’s not in the Pali
Canon at all. The same with dukkha-lakkhana and anatta-lakkhana: Those
compounds don’t appear. This is not to say that the concepts of anicca, dukkha,
and anatta don’t occur in the Canon; just that they’re not termed characteristics.
They’re not compounded with the word “characteristic.” The words they are
compounded with are perception, sañña—as in the perception of inconstancy, the
perception of stress, and the perception of not-self—and the word anupassana,
which means to contemplate or to keep track of something as it occurs. For
instance, aniccanupassana, to contemplate inconstancy, means to look for
inconstancy wherever it happens.
Now, it’s true that you’ll frequently find in the Canon the statements that all
things compounded or fabricated are inconstant, that they’re all stressful. And all
dhammas—all objects of the mind—are not-self. So if that’s the way things are,
why not just say that these are characteristic features of these things? Why make
a big deal about the language? Because words are like fingers, and you want to
make sure they point in the right direction—especially when they’re laying
blame, the way these three perceptions do. And in our practice, the direction
they point to is important for a number of reasons.
One is that the Buddha’s concern is not with trying to give an analysis of the
ultimate nature of things outside. He’s more interested in seeing how the
behavior of things affects our search for happiness. As he once said, all he taught
was suffering and the end of suffering. The suffering is essentially an issue of the
mind’s searching for happiness in the wrong places, in the wrong way. We look
for a constant happiness in things that are inconstant. We look for happiness in
things that are stressful and we look for “our” happiness in things that are notself,
that lie beyond our control. The three perceptions of inconstancy, stress, and
not-self are focused on our psychology, on how we can recognize when we’re
looking for happiness in the wrong way so that we can learn to look for
happiness in the right places, in the right ways. The contemplation of these three
themes, the use of these three perceptions, is aimed at finding happiness of a true
and lasting sort.'
http://www.dhammatalks.org/Archive/Writings/CrossIndexed/Published/Meditation4/070821%20Three%20Perceptions.pdf