Praise be to Allah.
One of the obligatory parts of wudoo' without which it is not valid is washing the arms from the fingertips to the elbow once, but it is mustahabb to wash them three times.
What is meant by once is that the water should reach all of the arm, no matter how it is done. If the water reaches all of the arm from the tips of the fingers to the elbows, then the arm has been washed once.
If the water reaches the arm a second time from the elbow to the fingertips and so on, then this is a second time.
See: al-Fawaakih al-Dawaani, 1/144.