This research study is a way of gaining new knowledge about the combination of Taselisib with other drugs in the treatment of metastatic breast cancer. Taselisib is an investigational drug which works by blocking a protein called PI3K (phosphoinositide 3-kinase) that helps cancer cells grow. This drug has been used in laboratory experiments and information from these studies suggests that this drug may help to prevent or slow the growth of cancer cells. The main purpose of this study is to find the appropriate dose of Taselisib to be used with other drugs in further clinical studies. This is an open-label, 3+3 dose-escalation phase Ib study to identify the Maximum Tolerated Dose(s) (MTD) and to identify the recommended phase 2 dose (RP2D) of Taselisib. This study will be conducted in 4 separate arms. (A-D).