5 days in Iceland in August, with Panasonic cameras

Hi!

Well, You don't need extra waterproof camera. Just use common sense during handling it. When is raining hevily, You can't make good photos anyway. Just hide it under your jacket (ready for use) or in Your backpack (with rain cover on it).
I was to Iceland many times (4 actually), on 3 long (>2 weeks) travels I used digital cameras: once Sony compact and then twice my E-PL1. They survived icelandic weather and even later mountain treks in polar Sweden -similar weather conditions.
You're going to Iceland for extremally short time (really a big mistake!). For sure You would be travel by car, doing only small trips on foot. It's not very challenging for your gear.
About lenses & co.: 14-140 II, 9-18 and 20 give all You need.  In summer there is almost no night in Iceland. So the tripod is of little use, unless You're exploring lava caves. Aurora can be seen in the very end of August only if  You're lucky. But You go for 5 days, probably in the middle of month... So, leave big tripod home, unless You need it for long exposure effect at waterfallls, streams etc. As You travel by car, You can also take it anyway. Or just take mini tripod -there are many rocks/stones You can put it on. Another topic possibly worth tripod (and longer tele lens) - birds on the cliffs. But if You go in late August they're mostly gone.
I see no sense of taking 3 camera bodies at all, unless You really want some IR pictures from Iceland of course.
One important thing: on 5 days trip You really have to limit yourself to the small part of Iceland only (mostly south coast). Some are trying to make a very basic circle around island in 7 days, but in my honest opinion it's ultimate stupidity. They mostly see the country through the car window only, having no idea what they're loosing.

Regards, and "enjoy Iceland/the silence" 

-J.