All of the medications that are being tested in this study are approved by the Food and Drug administration (FDA) for the treatment of metastatic breast cancer. However, the combination of these three medications in participants has not yet been tested. Eribulin is a chemotherapy agent that is approved for the treatment of metastatic breast cancer for women who have previously received at least two prior chemotherapeutic regimens for the treatment of their metastatic disease. Pertuzumab and trastuzumab are also both approved for the treatment of advanced HER2-positive breast cancer. Both agents help treat breast cancer by binding HER2 receptor. However, pertuzumab and trastuzumab bind to different parts of the HER2 receptor.