Involved in the biosynthesis of the Lewis X (LeX) trisaccharide of the lipopolysaccharide (LPS) O-antigen. Catalyzes the addition of fucose in alpha 1-3 linkage to Gal-beta-1-4-GlcNAc-beta-O-R (LacNAc-R) type II acceptor. a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + GDP-beta-L-fucose = a beta-D-galactosyl-(1->4)-[alpha-L-fucosyl-(1->3)]-N-acetyl-beta-D-glucosaminyl derivative + GDP + H(+) Lipopolysaccharide biosynthesis; LPS oligosaccharide biosynthesis. Homodimer. Predominantly localizes at the membrane. The tandem repeat region, which may form a leucine zipper structure, is essential for dimerization as well as for enzyme stability. Belongs to the glycosyltransferase 10 family.