During cellular respiration, one molecule of glucose (C6H12O6) is converted into carbon dioxide (CO2), water (H2O), and energy. The overall equation for the complete cellular respiration of glucose is:
\[ C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O \]
From this equation, we can see that one molecule of glucose produces 6 molecules of CO2.
If we have three glucose molecules undergoing cellular respiration, the total number of CO2 molecules produced can be calculated as follows:
\[ 3 \text{ glucose molecules} \times 6 \text{ CO2 molecules per glucose} = 18 \text{ CO2 molecules} \]
Therefore, the answer is:
18