Catalyzes the formation of 4-(hydroxymethyl)-2-furancarboxaldehyde phosphate (4-HFC-P) from two molecules of glyceraldehyde-3-P (GA-3-P). 2 D-glyceraldehyde 3-phosphate = 4-(hydroxymethyl)-2-furancarboxaldehyde phosphate + 2 H2O + phosphate Cofactor biosynthesis; methanofuran biosynthesis. Belongs to the MfnB family.