One flash in a stereo setup

DirkL wrote:

Wow, that was a comprehensive answer, thanks a lot

I intend to use a flash as fill flash outdoors. Yes, I noticed strange 3D shadows indoors with the W3 too, and I've read that using two camera flashes in 3D-macro might also result in problematic shadows. So I was wondering.

I am already using a remote control to trigger the cameras, maybe I am able to fire the flash too with it, have to look into it, but I don't think so.

No, if you use the same trigger for the cameras and the flash the flash will fire before the camera shutters open.  You need a second set of triggers.  With my 7D I use one set of RF triggers on one frequency to trigger the camera.  A second set of RF triggers operating on a different frequency are for the lights - one is mounted on the hot-shoe flash shoe, the other on one of my studio strobes.

But I also will need a second tripod or such, which will be awkward. And since the is plenty of available light, maybe the shadows will not be so harsh?

For my W3 I made a simple light reflector out of a piece of a plastic box, some aluminum foil, and a bit of glue and tape.  It has a tongue that fits between the camera body and the sliding lens cover to hold it in place over the flash.  The light from the built-in flash is directed upward where it bounces off the ceiling and then triggers my studio strobes.

Another way to use a built-in flash to trigger off-camera flash units is by using a fiber optic cable.  These are sold for underwater cameras but are expensive.  You can make your own for only a few dollars for non-underwater use.

ScubaBoard.com - my own DIY fiber optic trigger (for strobe)

PetaPixel - Trigger an External Flash with Some Fiber Optic Cable

If your flash doesn't have a built-in optical trigger then you could use the fiber optic cable with an optical hot-shoe flash trigger like this one.

B&H - Dot Line Hot Shoe Photographic Slave Flash Unit