Look at HELP for operator precedence.

Unless instructed by the use of parentheses, Excel will evaluate the negation
operator prior to the exponentiation operator.

In your first two examples, you are trying to computing the exponent of a
negative number.  My limited math background leads me to believe that the
example you give would result in an irrational number.  But I'm sure others
more knowledgeable will chime in

In your last example, you are squaring the negative number first (which gives a
positive number) and then taking the root of that positive number.

--ron