To find out how many friends were called on Sunday, we can break things down step by step:
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Friday: Sharon called 3 friends.
- Total friends called on Friday = 3.
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Saturday: Each of those 3 friends calls 3 different friends.
- Each of the 3 friends called 3 friends, so: \[ 3 \text{ friends} \times 3 \text{ calls per friend} = 9 \text{ friends called on Saturday} \]
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Sunday: Each of the 9 friends called on Saturday calls 3 different friends.
- Each of the 9 friends called 3 friends, so: \[ 9 \text{ friends} \times 3 \text{ calls per friend} = 27 \text{ friends called on Sunday} \]
Thus, the total number of friends called on Sunday is \( \boxed{27} \).