The Calvin cycle (C3 cycle) is the process by which plants fix carbon dioxide into organic molecules. To produce one molecule of glucose ($C_6H_{12}O_6$), which contains 6 carbon atoms, the cycle must fix 6 molecules of $CO_2$.
Since each turn of the Calvin cycle fixes one molecule of $CO_2$, it takes 6 turns of the cycle to produce one molecule of glucose ($C_6H_{12}O_6$).
If 1 molecule of glucose requires 6 turns, then 3 molecules of glucose will require: $$3 \times 6 = 18 \text{ turns}$$
Final Answer: 18
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