Use [ddd], [Ddd] or [DDD] to get a shortened version of the weekday name for the current date and current language. All three tokens give the same result. The only difference is the capitalization.
Note that this token does not necessarily abbreviate the weekday name to 3 letters. It displays whatever translation you have set for the [mmm] tokens in the Tokens Manager. For each script you can additionally specify that you wish to only have a limited number of letters (see the Length of [ddd] option in Grid Options).
So, on Tuesday October 12th 1998, in English, the result may be :
•[ddd] = tue,
•[Ddd] = Tue
•[DDD] = TUE.
The reason why we say the result may be, is that the above tokens are mixed source tokens. Q++ will decide, based on the current date, which day of the week it is, but it will then look for the translation that you supplied for 'Mon', 'Tue', 'Wed' … in the current language.
Since you specify the translations, you have total freedom. For example you may decide that in Swedish the shortened weekdays are not the first 3 letters (as in English), but the first 4 letters. Similarly you can easily create a descendant language based on English, where the [ddd] tokens are replaced by only the 2 first letters of the weekday name.
Q++ already comes with the standard abbreviations for weekdays in many languages.