(defn f1 [n]
  (cond
    (< n 3) n
    (+
     (f1 (- n 1))
     (* 2 (f1 (- n 2)))
     (* 3 (f1 (- n 3))))))

(defn f2-s [ a b c ]
  (+ a (* 2 b) (* 3 c)))

(defn f2-iter [ a b c i n ]
  (cond
    (= i 0) (cond
              (= n 0 ) a
              (f2-iter a b (f2-s a b c) (+ i 1) (- n 1)))
    (= i 1) (cond
              (= n 0 ) c
              (f2-iter a (f2-s c a b) c (+ i 1) (- n 1)))
    (= i 2) (cond
              (= n 0 ) b
              (f2-iter (f2-s b c a) b c 0 (- n 1)))))

(defn f2 [n]
  (cond
    (< n 3 ) n
    (f2-iter 2 1 0 0 (- n 2))))

(defn f3-iter [a b c n]
  (def new_c (f2-s a b c))
  (def new_b (f2-s new_c a b))
  (def new_a (f2-s new_b new_c a))
  (cond
    (= n 1) new_c
    (= n 2) new_b
    (= n 3) new_a
    (f3-iter new_a new_b new_c (- n 3))))

(defn f3 [n]
  (cond
    (< n 3) n
    (f3-iter 2 1 0 (- n 2))))

(print (f3 30))
(print (f2 30))
(print (f1 30))