While they look the same, the construction techniques are totally different. The ES339 is laminate top back and sides with a centerblock, the same as an ES335, just a smaller overall profile. The ES336 is a routed mahogany block with a carved maple cap, like a "hollow Les Paul" in a semihollow shape. Since they both will have maple on the top, its almost impossible to tell the difference from a distance or in a picture.
Gibson doesn't list the 336 in their lineup anymore, but the 339 is still made.