2 cubes each with side b are joined to form a cuboid . what is the surface area of this cuboid? is it 12b2? is the surface area of cuboid formed by joining three such cubes 18b2? why?

Surface area of Cube is 6*side*side. There are total 6 squares on a Cube, so on two different Cubes you have 12 squares. When you join two cubes of same side, then you will loosing two squares where the cubes are joined, because these surfaces become interior and hence cannot be called as surfaces. So now your Cuboid has 10 squares. So total surface area of the new Cuboid is 10*side*side :)