Decreasing the spacing between two staves

Version 3 is putting in space to avoid something else. You need to figure out what that something else is and then figure out how to ignore it, move it or delete it.

To be clear, in MuseScore 2, shift+drag was just a synonym for "extra distance above staff" - they did exactly the same thing. Which is, reduce the distance between that staff and next on each and every system, thus creating unequal distance between staves throughout your entire score. It would be quite unusual to want to do this - normally, the purpose of this action would be to create additional space between particular staves throughout your score, like between woodwinds and brass in an orchestral score. So already, I'm not really dsure what problem you are trying to solve, but chances are there was a better way to do whatever it was you were doing this way (like maybe you should have been simply reducing the staff distance in your style settings)?

Anyhow, for 3.0, while shift+drag no longer is available as a synonym for extra distance above staff, the latter works exactly the same as always, so whatever it could do before, it can now - except that you cannot use it to actually create collisions between the contents of the staves. If the goal is to deliberately overlap things, you can instead use the new fixed spacer.

In general, we can understand and assist better if you attach your score and explain the goal.