I have imported 3 wav’s in 3 different tracks. Now I want to increase the volume on only one track, but it increases on all 3 tracks.
What can I do???
Select only the track (or part of the track) that you want to apply the effect to. (see here for details: http://manual.audacityteam.org/o/man/audacity_selection.html )
That’s easy, thanks a lot 
Perhaps you can also tell me, if there is a smart way to adjust all 3 elements so they get the same volume?
That would be nice wouldn’t it
Unfortunately it is not so simple. “Loudness” is not a scientific measurement, it’s a subjective perception. How “loud” something sounds depends not only on how “big” the waveform is (amplitude) but also on the frequency content, whether the sound is a constant level or varies rapidly, on the hearing of the person listening and other factors. The only “accurate” way to get levels to match in terms of “loudness” is by listening to them and adjusting as necessary.
If you are adjusting the level of the entire track (not just part of a track, then you can use the “track gain slider” http://manual.audacityteam.org/o/man/audio_tracks.html#gain
This has the (big) advantage that you can adjust it while the track is playing.
If you set the “Solo button” behaviour to “Simple” (http://manual.audacityteam.org/o/man/tracks_preferences.html) then you can quickly switch between listening to one track and listening to another, simply by clicking the Solo button of the track that you want to listen to.
There is also the “Mixer Board” (http://manual.audacityteam.org/o/man/view_menu.html#mixer) which duplicates the track controls in a more convenient layout (warning, I’m not sure how stable this feature is - it used to be a bit prone to crashing on some machines, but I’ve not used it in a while so it may be OK now).
While using the track gain sliders (or mixer board), keep an eye on the meter levels so as to avoid pushing the levels too high.
Thanks a lot, but it seems to me that the best way is to use Effect–> Amplify on a single part