graph theory – Understanding equivalence classes

I am struggling to conceptualize and answer the following question. Consider the relation R that consists of all pairs (x,y) such that x and y are bit strings of length three or more that agree except perhaps in their first three bits. What are the equivalence classes, and how many are there?

I know that R is an equivalence relation, as it is Reflexive(all bits of x are the same as the bits of itself), symmetric(all bits of x(except perhaps the first three) are the same as all bits of y(except perhaps the first three)), and Transitive.

Any help is appreciated!