Inclusion Criteria:

• For this study neoadjuvant and/or adjuvant chemotherapy regimens do not count as a prior line of therapy
• For Cohorts A and C
  • At least one systemic treatment for metastatic breast cancer
  • Documented disease progression on the most recent therapy
  • Prior treatment must include an anthracycline and a taxane in the neoadjuvant, adjuvant, or metastatic setting
• For Cohort B
  • No prior systemic treatment for metastatic breast cancer
  • PD-L1-positive mTNBC
• For Cohort C
  • PD-L1 strong positive mTNBC
• For all cohorts
  • mTNBC confirmed by a central laboratory
• For biomarker analysis
  • adequate newly obtained core or excisional biopsy of a metastatic tumor lesion not previously irradiated
• Measurable metastatic disease
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Female participants of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication
• Male participants should agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy
• Adequate organ function

Exclusion Criteria

• Currently participating in and receiving study therapy
• Has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks prior to study Day 1
• Prior anti-cancer monoclonal antibody (mAb) therapy for direct anti-neoplastic treatment within 4 weeks prior to study Day 1
• Prior chemotherapy, targeted small molecule therapy, or radiation therapy within at least 2 weeks prior to study Day 1
• Not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents administered within at least 2 weeks prior to study Day 1
• Active autoimmune disease requiring systemic treatment in past 2 years
• Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study medication
• Known additional malignancy that has progressed or required active treatment within the last 5 years
  • Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of the skin that has undergone potentially curative therapy, or in situ cervical cancer
• Radiographically-detectable central nervous system (CNS) metastases and/or carcinomatous meningitis
• History of pneumonitis requiring treatment with steroids
• History of interstitial lung disease
• Active infection requiring systemic therapy
• Known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial
• Pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 120 days after the last dose of trial medication
• Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 agent or with an agent directed to another co-inhibitory T-cell receptor (e.g. CTLA-4, OX-40, CD137) or has participated in Merck MK-3475 trials
• Known history of human immunodeficiency virus (HIV)
• Known active Hepatitis B or C
• Received a live vaccine within 30 days of planned start of study medication