Here is the simple default strategy included with nim-client.rkt:
(define client-strategy
'(lambda (game-state) ;; Note the quote (')
(let ((c1 (size-of-pile game-state 1))
(c2 (size-of-pile game-state 2))
(c3 (size-of-pile game-state 3)))
(cond ((> c1 0) (make-move-instruction 1 1))
((> c2 0) (make-move-instruction 1 2))
(else (make-move-instruction 1 3))))))