>>If one side of a card is a vowel, the number on the other side is a number.

In most cases numbers are numbers.

By reversable I ment the rule. eg that an odd number has a vowel on the other side.

You can not assume that each card has a letter on one side and a number on the other.

Just as you can not assume that the cards do not have a face and a back (ie different sides like playing cards). If so then you would not need to turn the three as it has the 'wrong' side showing.

Yes, it all comes down to 'do I have to remove all chance of the rule being wrong'.

In which case you must turn two.

[to prove the 3 still fits the rule (ie has a number or vowel on the other side)]