A total of 18 molecules of oxygen (O2) are required for the aerobic respiration of three molecules of glucose. According to the balanced equation for cellular respiration, C6H12O6 + 6O2 → 6CO2 + 6H2O,1 molecule of glucose requires 6 molecules of oxygen. Therefore, for three molecules of glucose, 3 x 6 = 18 molecules of oxygen are needed.