Ex-1.11

;;
;; foo(n) = 0 if n=0
;;        = 1 if n=1
;;        = 2 if n=2
;;        = foo(n-1)+2foo(n-2)+3foo(n-3)
;;

;;recursive definition
(define (foo n)
 (cond
  ((= n 0) 0)
  ((= n 1) 1)
  ((= n 2) 2)
  (else
   (+ (foo (- n 1))
      (* 2 (foo (- n 2)))
      (* 3 (foo (- n 3)))))))

;;iterative definition
(define (foo n)
 (foo-iter 2 1 0 n))
(define (foo-iter a b c count)
 (cond
  ((= count 0) c)
  (else
   (foo-iter
    (+ a (* 2 b) (* 3 c)) a b (- count 1)))))

The general technique to convert recursive->iterative is to somehow find an invariant assertion that remains true from one iteration to another and wind-up the state in arguments.
As in above iterative definition state is in a, b & c and invariant assertion is that c, b & a are foo of 3 consecutive numbers.