Thank you Guloluseus for your prompt reply!
I am afraid I did not explain it very good..
Let me rephrase please.
I have a summary task with 3 phases.
phase 1 --> phase 2 --> phase 3

For my project I can control only when phase 2 starts.

The start date of phase 2, basically defines when phase 1 and 3 need to start. Phase 1 has to finish exactly when phase 2 starts. Phase 3 needs to start exactly when phase 2 ends.

I will probably be asked to change phase 2 start date many times, so I need my planning to be flexible and for phase 1 and 3 to adapt automatically to the changes.

So what I want to illustrate in my Gaant chart is that if i move the start of phase 2 by X days (in the past or the future), I would like both phase 1 and phase 3 to be adapted automatically and be shifted accordingly by X days.

Let me know please if you have advice on how to handle the dependencies for that.

Thank you
dnantia