In this study, the investigators are testing the effectiveness of the combination of eribulin, pertuzumab and trastuzumab to learn whether this combination of drugs works in treating advanced HER2-positive breast cancer that had received at least one prior treatment previously. At this point, the standard treatment for HER2-positive cancer that has progressed (grown) after a first treatment is chemotherapy combined with therapies that target the HER2 protein (e.g., trastuzumab or lapatinib).