Canon ST-E3-RT-V2 Manual page 26

a: Fully Automatic Wireless Flash Shooting
Autoflash Shooting Using Multiple Receiver Units
You can press the depth-of-field preview button on the camera to fire the

modeling flash (p.43).
If the receiver unit's auto power off takes effect, press the sender unit's

test flash button (p.13) to turn on the receiver unit. Note that the test flash
cannot be fired while the camera's metering timer is operating.
The autoflash system (E-TTL II/E-TTL) depends on the camera used and

is set automatically. Note that <a> is displayed on the LCD panel for
both systems.
You can enable a beep to sound when the charge of all the receiver units

is complete (C.Fn-20/p.59).
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When you need more flash output or you
want to perform lighting more easily, you
can increase the number of receiver
units and fire them as a single flash.
To add receiver units, use the same
procedure as "Autoflash Shooting Using
One Receiver Unit". Set A, B or C as the
firing group. The flash will not fire if it is
set to D or E.
When the number of receiver units is
increased, automatic control is
performed to fire all flashes at the same
flash output and ensure that the total
flash output results in the standard
exposure.