piece has the same basic shape as the previous two, but it's the smallest version.
begin by connecting two cubes in a line. then add one cube perpendicular to those first two, as shown in the second picture above. add another cube perpendicular to the plane of the first three cubes, as shown in the third image above. keeping symmetry in mind, add another cube opposite to the one you just glued, as shown in the fourth picture above. finish off the puzzle piece with one more cube to get what's shown in the fifth image above.
again, at this point you check that your pieces fit together: this one should fit 