Either one of two approaches I can think of:
1. Use an algorithm known as Zeller's Congruence to determine the day of today's date, if it's anything other than Tuesday, then determine the number of days between the current day and Tuesday and add that to today's date and return that date.
2. Use the storage format on the TDateTime type (or whatever "Now" uses = "x days/hours/minutes/seconds from this date") to determine that. You should be able to establish the day of the week that "this date" is, and adjust the number accordingly to establish the date you want to return.
If details are necessary, I can draw up some bits of code for this.