Regulatory component of the cyclin D3-CDK4 (DC) complex that phosphorylates and inhibits members of the retinoblastoma (RB) protein family including RB1 and regulates the cell-cycle during G(1)/S transition (PubMed:8114739). Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F complex and the subsequent transcription of E2F target genes which are responsible for the progression through the G(1) phase (PubMed:8114739). Hypophosphorylates RB1 in early G(1) phase (PubMed:8114739). Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals (PubMed:8114739). Component of the ternary complex, cyclin D3/CDK4/CDKN1B, required for nuclear translocation and activity of the cyclin D-CDK4 complex (PubMed:16782892). Shows transcriptional coactivator activity with ATF5 independently of CDK4 (PubMed:15358120). Interacts with the CDK4 and CDK6 protein kinases to form a serine/threonine kinase holoenzyme complex (PubMed:9106657, PubMed:19237555, PubMed:8114739). The cyclin subunit imparts substrate specificity to the complex (PubMed:8114739). Interacts with ATF5 (PubMed:15358120). Interacts with EIF3K (PubMed:15327989). Component of the ternary complex cyclin D/CDK4/CDKN1B required for nuclear translocation and modulation of CDK4-mediated kinase activity (PubMed:16782892). Can form similar complexes with either CDKN1A or CDKN2A (PubMed:9106657, PubMed:16782892). Phosphorylation at Thr-283 by MAP kinases is required for ubiquitination and degradation by the DCX(AMBRA1) complex. Ubiquitinated by the DCX(AMBRA1) complex during the transition from G1 to S cell phase, leading to its degradation: ubiquitination is dependent on Thr-283 phosphorylation (PubMed:33854235, PubMed:33854239). The DCX(AMBRA1) complex represents the major regulator of CCND3 stability during the G1/S transition (PubMed:33854235, PubMed:33854239). Polyubiquitinated by the SCF(FBXL2) complex, leading to proteasomal degradation (PubMed:22020328). Belongs to the cyclin family. Cyclin D subfamily.