What do you say regarding someone who, while performing ablution, becomes unsure whether he washed twice or thrice?

If one doubts the number of washes in ablution, whether he washed a limb three times or two, then he should perform the third wash as a precaution for what is recommended. This is not obligatory, for if he sufficed with a single wash, it would be sufficient for him. This does not lead to exceeding the forbidden three washes, because the prohibition of the fourth wash is conditioned upon being certain or thinking that one has performed the third, which has not occurred in this situation.