The participants of this study will have advanced prostate cancer known as metastatic castration resistant prostate cancer (mCRPC). The prostate is a small gland in the pelvis and is part of the male reproductive system. Prostate cancer is when abnormal cells form and grow in the prostate gland. In mCRPC the cancer stops responding to hormone treatment and is found in other parts of the body. Part 1 of this trial will study the safety and the level that adverse effects of each of the study drug combinations can be tolerated (known as tolerability). It is also designed to establish a recommended study drug dosage for the next part of the study. Part 2 will evaluate and compare how long participants live without their disease getting worse when receiving each of the study drug combinations. The study drug is call tazemetostat. The study will test tazemetostat in combination with either enzalutamide or abiraterone/prednisone.