Treatment with endocrine therapy (ET) in combination with CDK4/6 inhibitors has improved the outcome of patients with hormone receptor (HR)+/HER2- advanced breast cancer (ABC), but most patients eventually experience disease progression. Since the PI3K-AKT-mTOR (PAM), estrogen receptor (ER), and cyclin-dependent kinase (CDK) pathways are interdependent drivers of HR+/HER2- breast cancer (BC), the simultaneous inhibition of these pathways is expected to enhance anti-tumor control. Here we investigated the molecular and cellular effects of gedatolisib, a multi-target kinase inhibitor of the PAM pathway currently being evaluated in Phase 3 clinical trials, combined with fulvestrant and/or palbociclib in BC cell models. We found that the gedatolisib/fulvestrant/palbociclib triplet inhibited BC cell growth significantly more than the single agents or the palbociclib/fulvestrant doublet, both in vitro and vivo. Specifically, the triplet combination counteracted adaptive responses associated with single drug treatment, such as the reactivation of the CDK-RB-E2F pathway after palbociclib treatment, and inhibited multiple cellular functions, such as cell cycle progression, cell survival, protein synthesis, and glucose metabolism. The triplet combination was effective in treatment-naïve BC cell lines as well as in cell lines adapted to palbociclib and/or fulvestrant, regardless of genetic alterations. Our findings provide a mechanistic rationale for conducting clinical studies evaluating gedatolisib in combination with CDK4/6 inhibitors and ET in HR+/HER2- ABC.