Copying from MuseScore 2 to MuseScore 3

That's not quite accurate, all features are available whether automatic placement is enabled or not. However, automatic placement does do what it is supposed to do and correct the collisions that would have existed in MuseScore 2, so if you prefer to see those collisions, you will indeed need to uncheck automatic placement :-). Also, if you had performed lots of manual adjustments in useScore to work around those collisions yourself, those manual adjustments likely won't make sense any more, so you are usually best off choosing to reset the positions when importing the file and MuseScore gives you the choice. Otherwise, MuseScore will do its best to honor your old manual adjustments but results can be unpredictable.