This study is testing a new treatment for patients with a specific type of breast cancer called **PD-L1 positive metastatic triple negative breast cancer**. This cancer has spread from its original spot to other parts of the body. The study combines **high dose radiation therapy**, **pembrolizumab** (a type of medicine that helps the immune system fight cancer), and different **chemotherapy** options (paclitaxel, nab-paclitaxel, or carboplatin and gemcitabine). - **Key Points**: 1. Treatment involves radiation for 3 doses and cycles of pembrolizumab and chemotherapy every 21 or 28 days. 2. Participation includes CT and bone scans, blood samples, and biopsies. 3. Regular follow-up every 12 weeks after treatment. **Eligibility**: Must be 18 or older, have specific breast cancer characteristics, be able to consent, and meet health criteria. **Exclusions**: Previous chemotherapy for this cancer type, certain health conditions, or being pregnant/breastfeeding. **Goal**: To see if this combination treatment can help stop cancer from growing for at least 12 months.