Catalyzes the addition of GlcNAc or GlcUA monosaccharides to the nascent hyaluronan polymer. Therefore, it is essential to hyaluronan synthesis a major component of most extracellular matrices that has a structural role in tissues architectures and regulates cell adhesion, migration and differentiation. This is one of three isoenzymes responsible for cellular hyaluronan synthesis. [hyaluronan](n) + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N-acetyl-beta-D-glucosaminyl-(1->4)-[hyaluronan](n) + UDP N-acetyl-beta-D-glucosaminyl-(1->4)-[hyaluronan](n) + UDP-alpha-D-glucuronate = [hyaluronan](n+1) + H(+) + UDP The enzymatic activity depends on the availability of cytosolic levels of UDP-GlcUA and UDP-GlcNAc. Glycan biosynthesis; hyaluronan biosynthesis. Homodimers (PubMed:25795779). Forms heterodimers with HAS2 and HAS1 (PubMed:25795779). Travels from endoplasmic reticulum (ER), Golgi to plasma membrane (PubMed:26883802). Actives only when present in plasma membrane (By similarity). O-GlcNAcylation controls its membrane localization (PubMed:26883802). A rapid recycling of HAS3 between plasma membrane and endosomes is controlled by the cytosolic levels of UDP-GlcUA and UDP-GlcNAc (PubMed:26883802). O-GlcNAcylation increases the hyaluronan synthase activity, HAS3 stability and its plasma membrane residence. The concentration of UDP-GlcNAc controls the level of O-GlcNAc modification. Belongs to the NodC/HAS family.