Component of a Polycomb group (PcG) multiprotein PRC1-like complex, a complex class required to maintain the transcriptionally repressive state of many genes, including Hox genes, throughout development. PcG PRC1 complex acts via chromatin remodeling and modification of histones; it mediates monoubiquitination of histone H2A 'Lys-119', rendering chromatin heritably changed in its expressibility (PubMed:28596365). Within the PRC1-like complex, regulates RNF2 ubiquitin ligase activity (By similarity). Plays a redundant role with PCGF5 as part of a PRC1-like complex that mediates monoubiquitination of histone H2A 'Lys-119' on the X chromosome and is required for normal silencing of one copy of the X chromosome in XX females (PubMed:28596365). Component of a PRC1-like complex that contains PCGF3, RNF2 and RYBP (PubMed:28596365). Interacts with RNF2 (PubMed:28596365). Interacts with CBX6, CBX7 and CBX8 (By similarity). Interacts with BCORL1 (By similarity). Recruited by the non-coding RNA Xist to specific nuclear foci that probably correspond to the inactivated X chromosome. Combined disruption of both Pcgf3 and Pcgf5 causes embryonic lethality; there are no live progeny. Male embryos are detected at 9.5 and 10.5 dpc, but are smaller than normal. Female embryos are already extensively degraded at 9.5 dpc. Placentas from male embryos have some parietal trophoblast giant cells, but fail to form a labyrinth. Placentas from female embryos lack trophoblasts altogether and fail to form a labyrinth. Defects can be attributed to the observed lack of monoubiquitination of histone H2A 'Lys-119' and lack of Xist-mediated silencing of one copy of the X chromosome.