• Completion of informed consent prior to any study specific procedures. Consent may be done remotely.

• Patients must agree to tissue collection for correlative studies at the specified timepoints

• Male aged 18 years and above

• Histologically or cytologically confirmed prostate carcinoma

• Localized or regional high-risk disease as defined by at least one of the following features: Prostate specific antigen (PSA) \> 20 ng/mL, T3a or higher, grade group 4-5 (i.e. Gleason score ≥ 8) as per National Comprehensive Cancer Network (NCCN) Prostate Cancer Version 2.2020 for high risk or very high risk prostate cancer, and/or regional lymph nodes positive for prostate cancer

• Planned for definitive treatment of local regional prostate cancer using XRT and androgen ablation

• Willing to undergo ongoing medical castration to maintain testosterone levels of ≤ 50 ng/dL (≤ 2.0 nM) throughout systemic treatment or have undergone bilateral orchiectomy

• Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2. Patients must have adequate organ and bone marrow function measured within 7 days prior to treatment registration as defined below:

• Hemoglobin ≥ 10.0 g/dL

• Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L

• White blood cells (WBC) \> 3 x 10\^9/L

• No features suggestive of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) on peripheral blood smear

• Platelet count ≥ 100 x 10\^9/L

• Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN) (except for patients with known Gilbert's disease). (Note: In subjects with Gilbert's syndrome, if total bilirubin is \> 1.5 x ULN, measure direct and indirect bilirubin and if direct bilirubin is ≤ 1.5 x ULN, subject may be eligible.)

• Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase \[SGPT\]) ≤ 2.5 x institutional upper limit of normal

• Calculated creatinine clearance (Cockcroft-Gault Equation) ≥ 30 mL/min

• Serum Albumin ≥ 3.0

• Serum potassium ≥ 3.5 mmol/L

• Able to swallow study drugs whole as a tablet/capsule

• Patients who have partners of childbearing potential (e.g. female that has not been surgically sterilized or who are not amenorrheic for ≥ 12 months) must be willing to use two methods of birth control including adequate barrier protection during the study and for 4 months after last dose of niraparib, abiraterone acetate, and/or apalutamide administration. In addition men should not donate sperm during this period. Please note that the efficacy of hormonal contraception may be decreased if administered with niraparib, abiraterone acetate, and/or apalutamide

• Patient is willing and able to comply with the protocol for the duration of the study including undergoing treatment and scheduled visits and examinations including follow up

• Medications known to lower the seizure threshold must be discontinued or substituted at least 4 weeks prior to study entry