You can use your own digital format, for example:
return string.Format(CultureInfo.CreateSpecificCulture("nb-NO"), "{0:0.###}", decVal);
You can read about standard number formats and custom number formats.
EDIT:
To handle thousands separators, you can use:
return string.Format(CultureInfo.CreateSpecificCulture("nb-NO"), "{0:#,0.###}", decVal);
, SO-.
P.S.: @Luaan (0. ###);