According to the Hanafi school, it is a confirmed sunna to wash each foot three times. [Shurunbulali, Maraqi al-Falah; `Ala al-Din `Abidin, al-Hadiyya al-`Ala’iyya]
When using a large jug/container of water, depending on its size, one may be able to wash one’s foot thrice with a single scoop. This is achieved by pouring, then rubbing; pouring, then rubbing and so on. Wastefulness is blameworthy and one should be wary of this.
Washing under a tap would be akin to continual washing. One simply washes the foot three times and has fulfilled one’s duty. There is no need to remove one’s foot from under the tap and then put it back. This is excessiveness and uncalled for.
And Allah knows best.
