Changing values on boxes.
This refers to Musescore 3.0.1 running in MacOS Mojave version 10.14.2. and the value boxes found, for example, in the scaling under Format > Page Attributes and the position of hairpins and dynamics.
Normally, and in Musescore 2, deleting a digit from the right using the backspace (my Mac keyboard has no Del key) deletes the digit, so you can go backspace, new value. In Musescore 3, using the backspace first time puts a zero in place of the existing digit and the new value is not accepted. You have to move the cursor to the right, using the arrow key, then backspace again before entering the new value. It would appear to be a bug.