How to make extra adress fields instead of only three?

Three lines are shown to save space. After entering the last email address press 'Enter/return' key to get a new line. When there are more lines than displayed, you will see a scroll bar appear. You can also move the section up/down to show/hide lines. hover over the dividing section between composing are and the lower formatting bar to get a double headed arrow, then left click and hold down to grab the line. Move up or down as required. see images below as guide.