To determine how many CO2 molecules are produced when three glucose molecules undergo cellular respiration, we can refer to the general equation for cellular respiration:
\[ C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{energy} \]
This equation shows that one molecule of glucose (C6H12O6) produces 6 molecules of CO2.
If we have three glucose molecules, we can simply multiply the number of CO2 produced from one glucose molecule by three:
\[ 6 \text{ CO2} \times 3 = 18 \text{ CO2} \]
Therefore, the correct answer is:
18.