Can I open at Unity editor a few scenes at the same time?
Uhm, no, only one at a time :)
You can use EditorApplication.OpenSceneAdditive to load multiple scenes into the editor. Unfortunately they load into the current scene, so while it brings all your stuff together into the editor, it doesn’t solve the workflow of working on multiple related scenes.
What would be nice is multiple Hierarchy windows, one for each scene, with a shared Scene view, and the ability to edit and save each of the loaded scenes independently.
I just created a new feature request, vote for it if you want this implemented!