I presume you mean contains 3 elements, not contains three elements (as subgroups of are either trivial or contain infinitely many elements).

Now, subgroups of are all of the form (why?). Thus, there exists such that ( is the subgroup generated by ).

Basically, this all translates as "the elements you are looking for are elements such that " (not equal to 1 as H is a proper subgroup).

Can you find three such elements?