I use a Guarder gun bag which can hold a rifle, smg and pistol, along with a couple of extra pockets for mags etc. It has straps for carrying like a holdall or on yoyr back like a rucksack.
If you're going to be using public transport I'd advise against a hard gun case as I found it attracted more attention, usually from pissed up office workers who thought they were funny asking "what you got in there, a gun?" Although the looks on their face when I snapped back "no, three of them" was often quite amusing