Vipassana Meditation

If I'm meditating for example with music and I am aware of the changes of the music that is impermanence right? If I breathe in and out that is impermanence? However, how do I see dukkha and not self (especially not self)? Dukkha can be a scratch for instance? One last question: In Vipassana one experiences all three characteristics, but does one also contemplate on all threes or is that a seperate meditation? Thanks in advance!