The purpose of this study is to determine the maximal tolerated dose (MTD) or recommended phase 2 dose (RP2D) of tucatinib (ONT-380) and to assess the safety and tolerability of tucatinib (ONT-380) combined with capecitabine alone, trastuzumab alone and with both capecitabine and trastuzumab in patients with HER2+ metastatic breast cancer.