divide the set into subset of two elements like(A,B),(B,C),(C,D),(D,E),(E,F),(F,G),(G,H),(H,I )

now find the corresponding ways for which u can choose the third element

1.(A,B):8

2.(B,C):7

3.(C,D):6

4.(D,E):5

5.(E,F):4

6.(F,G):3

7.(G,H):2

8.(H,I):1

now sum all these u get 36 as no of three letter words that can be formed