Let's calculate the sum of the perimeters of three small circles: suppose the diameter of the first small circle is A, the diameter of the second small circle is B and the diameter of the third small circle is C, then the circumference of the first small circle is A*3. 14.
The circumference of the second small circle is: B*3. 14.
The circumference of the third small circle is: C*3. 14.
Add them up: a * 3.14+b * 3.14 *+c * 3.14 = (a+b+c) * 3.14.
Because "the centers of these small circles are on the same diameter", that is to say, the sum of the diameters of the three small circles is on the diameter of the big circle, that is, A+B+C is the diameter of the big circle, then A+B+C)*3. 14 is the diameter of the big circle multiplied by 3. 14, so it is not 12.