Is it normal for a conference to let the submissions be editable on easychair website by the authors (and still be able to submit new papers) after 3 weeks of the hard deadline and only one week before the notification of acceptence? I thought they will disable these options once they assign the reviewers.
This is not normal.
EasyChair has many options that can be set by conference organizers. In principle, this is a good thing, since it allows the program chairs to configure their conference management system exactly as they want to. However, it also means that it is relatively easy to make mistakes in the EasyChair settings. I think the program chairs may simply be making a mistake right now, and they may not be aware that submissions can still be added/modified.