Catalyzes the addition of a mannose residue from GDP-D-mannose to the position 6 of the alpha-1,6-linked mannose residue of the triacyl phosphatidylinositol dimannoside (Ac3PIM2) to generate triacyl phosphatidylinositol trimannoside (Ac3PIM3). Phospholipid metabolism; phosphatidylinositol metabolism. Belongs to the glycosyltransferase group 1 family.