(I assume this is about the dash that replaces the author's name when successive publications have the same author.)
Key in three em dashes. They will have a short space between them. Select all three and choose Format>Font (command-D). In the Advanced tab of its dialogue box, select Condensed spacing to get rid of the space. One point condensed should be enough in a typical bibliography font and size, but you can use more if you need to.