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!
Do not meditate with music. It's distracting. Go to a quiet place with the least number of distractions.

There's no such thing called 'breathing'. Breathing is a concept. What you should focus on is the feeling at the nose when the air hits inside. Do not make more than what the senses can feel. All that is felt is the cold, warm, smooth or rough sensations rising and falling. Simply keep noting that. If you happen to hear something, just note it as hearing. Don't make more out of it. Same attitude with any other sensation or thought.

Do not try to thinkup impermanence, suffering and non-self. That is like digging a hole in the ground, putting water to it and calling it a water well. The proper way is to keep digging until you find water. In the same way, keep noting the sensations/thoughts and the 3 characteristics will reveal themselves when the mind is matured.

Here is a handbook for you: https://www.sirimangalo.org/text/how-to-meditate/