MuseScore 3 adds a new Insert mode to the dropdown list. In this mode, as you enter notes, they are inserted at the cursor position, moving the rest of the contents of the measure to the right and increasing the duration of the measure. You can use this to create unmeasured music. Tools / Remove Selected Range (Ctrl+Delete) will delete a note or rest from a measure, while in Normal mode. For more information, see the lesson on Inserting and Deleting Notes.