The way I read it I agree with your solution. It should be .027.

However, if the book intended that "the extension consists of the digits 1, 2, and 3 in any order" means that you need to have each of the digits 1, 2, and 3 in the number itself then the "repetitions are allowed" is just superfluous and slightly misleading but not incorrect and the book's solution is correct.

However, I'm going to say you're right and the book is wrong.