How many turns of Calvin cycle are required for the formation of...

One turn of the Calvin cycle fixes one molecule of $CO_2$.

To synthesize one molecule of glucose ($C_6H_{12}O_6$), six molecules of $CO_2$ are required.

Thus, six turns of the Calvin cycle are required to produce one molecule of glucose.

For the formation of three molecules of glucose, the number of turns required is $3 \times 6 = 18$.

Answer: $18$