Nobody will notice that the wall is 3” shorter on that side of the room.
Floor leveller is not going to help with the increasing tilt of the entire building. In fact, it will make the problem worse by adding weight to the side that is sinking.
The developer and all involved (especially the building inspectors and foundation engineers) with the construction should be held personally responsible and the building needs to come down before it collapses.
This goes all the way back to the original soil borings.
The developer probably did the minimum necessary (if that) to save a little money—penny wise and pound foolish.