Pouring water three times is Sunnah, not three distinct obligations. Once the complete obligatory surface has been washed, the obligatory washing is fulfilled; the second and third washings complete the Sunnah.
A dry obligatory area must still be washed, regardless of how many pours were made.
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Ghusl: Obligations, Sunnah Procedure and Braided Hair
Pouring water three times is Sunnah, not three distinct obligations. Once the complete obligatory surface has been washed, the obligatory washing is fulfilled; the second and third washings complete the Sunnah.
A dry obligatory area must still be washed, regardless of how many pours were made.