Accessory component of both the DNA polymerase delta complex and the DNA polymerase zeta complex (By similarity). As a component of the trimeric and tetrameric DNA polymerase delta complexes (Pol-delta3 and Pol-delta4, respectively), plays a role in high fidelity genome replication, including in lagging strand synthesis, and repair (PubMed:10219083, PubMed:27524497). Required for optimal Pol-delta activity. Stabilizes the Pol-delta complex and plays a major role in Pol-delta stimulation by PCNA. Pol-delta3 and Pol-delta4 are characterized by the absence or the presence of POLD4. They exhibit differences in catalytic activity. Most notably, Pol-delta3 shows higher proofreading activity than Pol-delta4. Although both Pol-delta3 and Pol-delta4 process Okazaki fragments in vitro, Pol-delta3 may also be better suited to fulfill this task, exhibiting near-absence of strand displacement activity compared to Pol-delta4 and stalling on encounter with the 5'-blocking oligonucleotides. Pol-delta3 idling process may avoid the formation of a gap, while maintaining a nick that can be readily ligated. Along with DNA polymerase kappa, DNA polymerase delta carries out approximately half of nucleotide excision repair (NER) synthesis following UV irradiation. In this context, POLD3, along with PCNA and RFC1-replication factor C complex, is required to recruit POLD1, the catalytic subunit of the polymerase delta complex, to DNA damage sites. Under conditions of DNA replication stress, required for the repair of broken replication forks through break-induced replication (BIR). Involved in the translesion synthesis (TLS) of templates carrying O6-methylguanine or abasic sites performed by Pol-delta4, independently of DNA polymerase zeta (REV3L) or eta (POLH). Facilitates abasic site bypass by DNA polymerase delta by promoting extension from the nucleotide inserted opposite the lesion. Also involved in TLS, as a component of the tetrametric DNA polymerase zeta complex. Along with POLD2, dramatically increases the efficiency and processivity of DNA synthesis of the DNA polymerase zeta complex compared to the minimal zeta complex, consisting of only REV3L and REV7 (By similarity). Component of both the DNA polymerase delta and DNA polymerase zeta complexes (By similarity). The tetrameric DNA polymerase delta complex (Pol-delta4), which consists of POLD1/p125, POLD2/p50, POLD3/p66/p68 and POLD4/p12, with POLD1 bearing DNA polymerase and 3' to 5' proofreading exonuclease activities (PubMed:27524497). Within this complex, directly interacts with POLD2. Following stress caused by DNA damaging agents or by replication stress, POLD4 is degraded and Pol-delta4 is converted into a trimeric form of the complex (Pol-delta3), which consists of POLD1, POLD2 and POLD3. Pol-delta3 is the major form occurring at S phase replication sites, as well as DNA damage sites. Directly interacts with PCNA, as do POLD1 and POLD4; this interaction stimulates Pol-delta polymerase activity. POLD3 phosphorylation at Ser-454 impairs PCNA binding. Component of the DNA polymerase zeta complex (POLZ), which consists of REV3L, MAD2L2, POLD2 and POLD3, with REV3L bearing DNA polymerase catalytic activity. The DNA polymerase delta complex interacts with POLDIP2; this interaction is probably mediated through direct binding to POLD2 (By similarity). Partially colocalizes with PCNA and POLD1 at S phase replication sites. Recruited to DNA damage sites within 2 hours following UV irradiation. The PIP-box mediates the interaction with PCNA. Ubiquitinated, but not targeted to the proteasome. Sumoylated. Sumoylation by SUMO3 may be predominant. Phosphorylation at Ser-454 is thought to decrease the affinity for PCNA and Pol-delta4 processivity. May be phosphorylated by CDK1-cyclin-A complex, as well as CDK2-cyclin-A and CDK2-cyclin-E complexes. PCNA interferes with CDK-cyclin phosphorylation. In a conditional knockdown, embryonic lethal when homozygous. Induction of the knockdown in adult animals results in a generalized accumulation of DNA damage and ultimately death of all mice within 15 weeks after the beginning of the treatment. Heterozygous animals are born at sub-Mendelian ratios, and, of those born, some are dwarfs, present hydrocephaly and have a reduced lifespan. In cells, Pold3 deficiency leads to replication stress and cell death. Extended N-terminus.