Conjoined twins in a triplet pregnancy are very rare. Failure to diagnose a thoracopagus in a case of known triplet pregnancy resulted in obstructed labor which necessitated an emergency cesarean section. The single infant survived, but the thoracopagus died apparently from asphyxia.
Tan K, Tock EPC, Dawood MY, Ratnam SS. Conjoined Twins in a Triplet Pregnancy. Am J Dis Child. 1971;122(5):455-458. doi:10.1001/archpedi.1971.02110050125022