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

Click the "Show 3 dart average" checkbox and the close the form.

The average is switched instantly to three dart average. The setting is saved to registry and the default will be three dart average.