There are four mandatory ( farz ) acts of Wuzu,
Washing the arms ( from hands to elbows )
Washing the face
Wiping the head with wet hands
Washing the feet, including heelsIt is fard to wash the arm, face and feet at least once during wudu. If they are washed three times, then the sunnah is also fulfilled.
These acts are mandatory. If you will not perform or miss any one of these, then its Wuzu is not valid.