Here's what Ive done in the past. I got a large waist pack and was able to fit in my small video camera and small camera, autograph book, pen, wallet, ticket, snacks, small thing of bubbles, and some small things that I bought. If I got tired of wearing it I wore it like a purse (and did so on rides that had lap bars) or hung it from the stroller handles. I left the diapers, wipes, and jackets in our stroller basket and they never disappeared.
If you want a backpack, try the Healthy back bags. They have lots of little compartments and come in several sizes from mini to extra large. JJ Cole makes a neat bag called the Essentials bag. Its like a sash you wear across your chest. It doesnt look like much, but I got quite a bit in there when using it as a diaper bag. There are many pockets-some big enough for diapers, wipes, and change of clothes, to tiny for coins. It might not be big enough for 3 kids, but you can leave some stuff in the stroller.
This trip I will either use that bag or a mini backpack. I have a 4 yr old and a 1 yr old. The diaper supplies, ponchos and snacks will stay in the stroller and only the valuables will go with me since I will have to use a carrier for my 1 yr old.
alison