On my computer, I have partitions on my disk. One for windows (I believe some Gos), on other for shared documents(100 Go) and one for ubuntu (140 Go). It is not normal that when I use Tails, I can have accessibility to the both first partitions and also permissions to modify and create directories or files. Only the last is protected.

I have a hard time understanding what you mean: can you or can you not modify files on these partitions? And what is the problem?
Please also tell use which filesystem these partitions use (I suspect either FAT32 or NTFS).
Protected how?
Thank you for your answer and sorry for my english.
So if I have well understand the goal of a live system, usually you don't have acessibility to the hard disk. With a system live you write only on the RAM.
What I said is I can write on my windows partition (NTFS) because I can create modify and delete files.
Sorry if I have bad understood the function of the system tails. I hope you understand what I mean.
Tails only lives in RAM, but if you access a permanent storage such as a USB drive or one of your hard drives with intention (e.g. by opening one through the "Places -> ..." menu) then we assume that the you know what you are doing and allow that to happen. If you don't intentionally open a storage media, Tails will leave it untouched.