Find total number of triplet a,b,c

Given two array A and B. Both have size n. Each element of both array are from 1 to n and there are no duplicate element in any of the array. Means both array has same element but they are shuffle.
** for example: A={1 ,2 ,3 ,4 ,5 ,8 ,6 ,7} , B={ 5, 6 ,7 ,8 ,3 ,4 ,2 ,1 }**

What is the total number of unordered triplets (a, b, c) such that the relative ordering of (a, b, c) is the same in both the arrays.

one solution i thought is to find longest common subsequence and from that LCS find all possible a,b,c.
Is it my approach is right or not? if not than plz suggest optimized approach.

if you got the answer, please share.