If you have BO3 set, your end result will either be 2-0 or 0-2.
If you have BO5 set, your end result will either be 3-0 or 0-3.

Alternatively to these, you can set to 1, 2 or 3.  If set, to 3 for example, you then have the possibility of winning 2-1, 3-0 etc.

I hope this clears it up for you.