Final answer:
The third step in implementing an ABC system is to calculate activity rates by recording and calculating the average energy use by males and females during a 24-hour period.
Step-by-step explanation:
In implementing an ABC system, the third step is to calculate activity rates. This involves recording the average energy use by 10 males and 10 females during a 24-hour period and calculating the average energy use for both groups. This step helps determine the rates at which different activities consume resources.