Part C will investigate the efficacy of the triplet combination at the RP2D in a randomized, double-blind, placebo-controlled, stratified, parallel group extension. Part C will include ER+, locally advanced and/or metastatic breast cancer patients who have progressed following prior non-steroidal aromatase inhibitor (NSAI) endocrine therapy. Patients in Part C will be randomized to receive either the triplet combination (AZD2014 + palbociclib + fulvestrant) or the doublet (matching AZD2014 placebo + palbociclib + fulvestrant). Patients will be stratified according to hormone sensitivity, presence of visceral metastases, and prior CDK inhibitor treatment. Part C will be conducted if indicated by the emerging data.