Thanks. I missed that part of the notes

You can't change the numbers position, so the tests are fine. It's in the kata description:

You cannot swap the operands. For instance, in the given example you cannot get expression (1 + 3) * 2 = 8. Example being 1, 2 and 3.

Some of sample tests are wrong. "(a+c)*b" was forgotten. For example the result in the sample code((expression_matter(1, 8, 3), 27)) is 27. But the maximum result is 32((1+3)*8).