(define (sum-square a b) (* (+ a a) (+ b b)))
(define (sum-square-max a b c)
        (cond ((and (< a b) (< a c)) (sum-square b c))
              ((and (< b a) (< b c)) (sum-square a c))
              ((and (< c a) (< c b)) (sum-square a b))))

updated after meeting:

(define (sum-square a b) (* (+ a a) (+ b b)))
(define (sum-square-max a b c)
        (cond ((and (< a b) (< a c)) (sum-square b c))
              ((and (< b a) (< b c)) (sum-square a c))
              ( else                 (sum-square a b))))