Answer:
15
Explanation:
You want the sum of the numbers in the diagram that belong to exactly 3 circles.
Elimination
We find it easier to eliminate numbers that do not belong to 3 circles. In the attachment, the red lines strike through numbers belonging to 1 circle, the blue lines strike numbers belonging to 2 circles, and the green line strikes the number belonging to 4 circles. (The numbers inside the smallest circle are also in the larger circle surrounding it.)
The numbers that are in 3 circles only are 1, 3, 11. Their sum is 15.