It depends what you want to do. A pure shell element is virutally a "block". The third direction is included in the mathematical assumptions. If you want to model the same block you should model the length and width as surface. The thickness is then defined as parameter for the shell element.

It depends on whether the problem can be reduced to a 2D problem. Shell elements are used when one can safely afford to neglect the effects of the third direction and when the other two (1 and 2)directions are much larger comparatively as this saves time and number of elements.

Thanks for your replays,

Take an example I modeled a beam in a modeling software and later, I imported to any analysis software Now it is 3-D problem, can we solve this problem using Shell element