First, wear clothes within your comfort level. If you're not comfortable in a 3pc suit, don't try to pull it off because you'll look and feel awkward all night.
Second, I am neither Ruth nor Chris, but to me, smart casual is khaki's and a button down shirt. You can substitute good fitting jeans, but I would add a casual sports coat to finish the look.
I've been to Ruth's Chris several times and at no time did I feel like they were all about a dress code so relax and enjoy one of the finer steaks you'll find.