To determine how many CO2 molecules are produced when three glucose molecules undergo cellular respiration, we can look at the overall reaction for cellular respiration. The cellular respiration of one glucose molecule (C6H12O6) produces 6 CO2 molecules.
So, if three glucose molecules are used, the total CO2 produced would be:
3 glucose molecules × 6 CO2/glucose = 18 CO2 molecules
Therefore, the answer is 18.