@PeterTse :
After a little research I found that the 3rd Saturday of any month must be >= the 15th of the month and <= the 21st of the month. I created the following Flow as a test and the Logic appears to work.
The expressions used in the expression are dayOfWeek and dayOfMonth. For dayOfWeek, I supplied 6 for Saturday.
You can use this logic after a recurrence trigger that runs daily.

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Scott