Catalyzes the first step in the biosynthesis of chondroitin sulfate, heparan sulfate and dermatan sulfate proteoglycans, such as DCN (By similarity). Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein (By similarity). L-seryl-[protein] + UDP-alpha-D-xylose = 3-O-(beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Active with either Mg(2+) or Mn(2+), but activity is highest when both are present. Glycan metabolism; chondroitin sulfate biosynthesis. Glycan metabolism; heparan sulfate biosynthesis. Monomer. Contains disulfide bonds. Belongs to the glycosyltransferase 14 family. XylT subfamily.