Microtubule-based molecular motor that transport intracellular cargos, such as vesicles, organelles and protein complexes. Uses ATP hydrolysis to generate force to bind and move along the microtubule (PubMed:7559760). Plays a role in cilia formation (PubMed:9865700). Involved in photoreceptor integrity and opsin trafficking in rod photoreceptors (By similarity). Transports vesicles containing N-methyl-D-aspartate (NMDA) receptor subunit GRIN2A into neuronal dendrites (PubMed:31746486). Heterodimer of KIF3A and KIF3B (PubMed:7559760). KIF3A/KIF3B heterodimer interacts with KIFAP3 forming a heterotrimeric (KIF3A/KIF3B/KIFAP3) complex (PubMed:8710890). Interacts with the SMC3 subunit of the cohesin complex (By similarity). Interacts directly with IFT20 (PubMed:12821668). Interacts with FLCN (By similarity). Colocalized with GRIN2A in dendritic shafts and in DLG4-positive spines. Deficient mice do not survive beyond midgestation, exhibiting growth retardation, pericardial sac ballooning, and neural tube disorganization (PubMed:9865700). Kif3b +/- mice exhibit schizophrenia-like phenotypes, both behaviorally and histologically. Hippocampal neurons have altered spine morphology and synapse function, and at the cellular level, they display abnormal growth cone morphology (PubMed:31746486). Belongs to the TRAFAC class myosin-kinesin ATPase superfamily. Kinesin family. Kinesin II subfamily.