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Yes, the 90ex can trigger 3 groups of flashes. It's not as versatile as a st-e2, but works great on my EOS M and 5d2. I've posted below a pic of the setup on my 5d2 (taken with the M) and a couple of samples. The dollar shots are both at 5x, but one is at 3.2 (almost no DOF) and the second is at f13 (where diffraction is kicking in). For any real DOF, you need to take multiple shots and stack, which I am not very good at. For comparison, I also added a shot fo nickel at 5x and 2x.
5x at f13:
2x at f 11:
These are just quick examples I threw together to show the lighting. It's not perfect, but works fine for still subjects. This wouldn't work well for insects. I also usually use the rail on a full tripod so I can get complete angle control. That little manfrotto tabletop can't really handle it, but I wasn't going to break everything out...
I've done this with two lights using onboard commander and my 320, 430 or 580. I've used a bracket when handholding, but I find totally off camera almost as easy, even if holding the flash in my hand and balancing off it. Of course, there are far better shooters than me on here with more sophisticated solutions. This works for me because all of the gear (but for the rail) is multipurpose.
Hope this is helpful.