IMO 2 x .120 HREW is generally considered more than adequate for a full size cage in a rock crawler. If you do sand or Baja type stuff, you probably want to go smaller and stronger with DOM or Molly for weight savings AND the ability to hold up to 70+ mph rolls. If your talking just "roll bar" then I don't know about that. Most "roll bars" are really just fancy light hangers and, while better than nothing, are not designed to be "used" in a roll over. If your trying to build a stronger “roll bar” that would actually be semi-functional then I would go ahead and get a cage. Not that much more trouble, and MUCH more functional. Heck, you can buy a pre-bent cage from S&W now for what a decent roll bar will cost.
Edit: Oops, just reread your original post. Looks like you are talking about a cage after all and probably not interested in buying a S&W. 3" is way too big IMO. I'm leaning hard toward 1.75 x .120 to build a cage for myself.