MindSumo Launches Innovation Challenge: Technical Puzzle #44

Let f (x) denote the sum of the digits of the positive integer x. For example, f (8) = 8 and f (123) = 1+2+3 = 6. For how many two digit values of x is f (f (x)) = 3? Enter the just number of possible x values (Hint, the number is between 3 and 10, inclusive)

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