I normally use 2 rods, but when I decide to use 3 or more, I put whatever I'm not carrying in my hands into a soft rifle bag that I bought on the cheap. This, of course, only works with 2-piece rods, but it IS effective.

Simple, I don't. One at a time.

I bring one or two depending on where I'm fishing and what I'm planning on using.  The backpack I use is able to accomodate a rod in one of the two water bottle holders on the side so I lash one down there while I fish one rod.  Anything more than that and I've found it's too much of a hassle juggling rods around, where it actually impacts my fishing.

 

One product I saw announced this past iCast was the Rod Runner in a 3-rod configuration (previously only available in a 5-rod configuration) but haven't seen any reviews on it yet.

I usually only carry a 7' medium baitcaster and a 7' medium spinning rod when fishing from the bank. I have never found the need for anything other than those two.