All of the mentioned three are used in industry, all are fairly simple to use, with Inventor, in my opinion, being the easiest to learn, followed by SolidWorks and finally CATIA V5. If You learn one, to transfer knowledge to another one won't be difficult as all come with How To? tutorials (in my opinion CATIA V5 has the best How To?). CATIA V5 has more rustic GUI where Inventor and SolidWorks has more modern one (and more intuitive, at least for me).