Something

(define (middle-of-3 x y z)
  (cond
    [(or (and (<= x y)(<= z x))(and (>= x y)(>= z x))) x] ;; x is in the middle
    [(or (and (<= y x)(<= z y))(and (>= y x)(>= z y))) y] ;; y is in the middle
    [else z] ;; if x and y both aren't in the middle than z is
   )
)