Cellular respiration converts glucose (C6H12O6) into carbon dioxide (CO2), water, and energy. The balanced equation for cellular respiration can be summarized as follows:
\[ \text{C}6\text{H}{12}\text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2\text{O} + \text{energy} \]
From the equation, we see that one molecule of glucose produces six molecules of carbon dioxide. Therefore, if three glucose molecules are used, the total amount of CO2 produced would be:
\[ 3 \text{ glucose} \times 6 \text{ CO}_2/\text{glucose} = 18 \text{ CO}_2 \]
So the correct answer is 18.