Consider the following premises. t <-> r_3. s <-> r_3. ~s. Can we infer ~t from them?
 From s <-> r_3 we can infer r_3 -> s. From r_3 -> s, ~s we can infer ~r_3. Finally, from t <-> r_3, ~r_3 we can infer ~t. Therefore, the answer is yes.