Help with multiple groups with T7i

[tedsled]

Sorry, originally posted in the wrong forum.  My bad.

I have a T7i and three third party speedlites (Voking V580) that support ETTL and optical wireless triggereing.  I am totally new to this so be gentle!

I am trying to set up the three V580s as off camera slaves triggered by the inbuilt camera flash.  If I put all three in Group A they all fire.  If I put one or two in Group A and the other(s) in Group B they all fire and I can control the ratio and exposure comp from the camera.  However, if I configure one V580 for Group A, one for Group B and the third for Group C, only A and B fire. C does nothing. 

My goal is to have two V580s used for side fill and the third as a highlight on the background.  Ideally, I'd like to be able to control the output for each individually, thus being able to adjust the left and right highlights as well as the backlight all individually.

What am I doing wrong?  I've read every relevant page in the T7i manual and the entire V580 manual multiple times now and I'm still clueless.   I searched for a possible tutorial or sticky that might help me, but came up empty.  If there is such a thing, lemme know and I'll read it.  I saw that Speedlite Book but oh my they want real money for it!!  More than all three V580s put together!!

Thanks for the help!

ted

[MikeSowsun]

Group C can be confusing when using the Canon proprietary system.  

This is from the Speedliter's Handbook by Syl Arena and may help. 

[p4pictures]

This is a feature of your camera. It can only control A and B groups off-camera. The only cameras that control C group using the pop-up flash are EOS 7D and EOS 7D Mark II.

But there might be a workaround… on Canon Speedlites you can put a slave in to so called individual slave mode. In this mode you have to manually set the power on the Speedlite itself, and it won't be in any group. However it will flash when the camera instructs flashes to fire. 

Normally a flash expects it's group letter, then a command, but when set to individual slave mode the only thing it reacts to is the "flash now" command.

A little more about this from an old post on my site: https://www.p4pictures.com/2014/05/speedlite-manual-mode/ 

I don't know if your Voking V580 supports individual slave mode.