The one dart average is the default setting for Darts for Windows 2008. A lot of players and tournament organisators are more used to see the 3 dart average. Switching from one dart average to three dart average is straightforward and it's activated immediately. To switch do the following:
Go to the menu "Settings" and click on "Options".

Scroll down to the bottom of the listbox as shown below. Click the "Show 3 dart average" checkbox and the close the form by clicking on the green arrow in the top left corner.

The average is switched instantly to three dart average. The setting is saved to registry so the program will continue to show three dart average for as long as you like.