What are the instances in which it is permissible to perform wudu once for each limb?

The Prophet, peace and blessings be upon him, performed ablution once, twice, and thrice, and scholars have clarified that this demonstrates the permissibility of these different manners. The obligatory act is to wash once, while washing three times is a Sunnah. Limiting oneself to once or twice is to show permissibility, or due to scarcity of water, or haste. This does not mean that it is only permissible in these situations; rather, the most complete way is to perform ablution three times.