Catalyzes the hydrolysis of beta-1,3-xylan into oligosaccharides, mainly xylobiose, xylotriose and xylotetraose. Converts beta-1,3-xylotriose into xylose and xylobiose, converts beta-1,3-xylotetraose mainly into xylotriose and xylose, converts beta-1,3-xylopentaose into xylobiose and xylotriose. Does not hydrolyze beta-1,4-xylan, beta-1,4-mannan, beta-1,4-glucan, beta-1,3-xylobiose or p-nitrophenyl-beta-xyloside. Random hydrolysis of (1->3)-beta-D-glycosidic linkages in (1->3)-beta-D-xylans. Completely inhibited by Hg(2+), partially inhibited by Mn(2+), Cu(2+) and Pb(2+). Unaffected by Ca(2+), Mg(2+) and EDTA. Optimum pH is 7.0-7.5. Optimum temperature is 37 degrees Celsius. Inactive above 60 degrees Celsius. The carbohydrate binding modules (CBM) bind to insoluble beta-1,3-xylan, but not to insoluble beta-1,4-xylan, beta-1,4-glucan, beta-1,4-mannan, curdlan, chitin, or soluble polysaccharides. Belongs to the glycosyl hydrolase 26 family.