Definition of power functions
Major

All 3 of your functions are linear (they do O(n) multiplications) while, in fact, only O(log(n)) is necessary:

(defun pow (x n)
  (if (minusp n)
      (/ (pow x (- n)))
      (multiple-value-bind (d r) (floor n 2)
        (let ((p (if (zerop d) 1 (pow x d))))
          (* p p (if (zerop r) 1 x))))))


In addition to much fewer multiplications, I am returning ratios for rational x.

Minor

Use minusp instead of comparison with 0.0 and (/ a) instead of (/ 1 a).