Canon Speedlite 600EX II-RT Manual (en)

a: Fully Automatic Wireless Flash Photography
When you need more flash output or you 
want to perform lighting more easily, you 
can increase the number of slave units 
and fire them as a single flash.
To add slave units, perform the same 
procedure as “Autoflash Photography 
Using One Slave Unit” (p.96). You can 
use any of A, B, or C as the firing group.
When the number of slave units is increased or master flash firing is set 
to ON, automatic control is performed to fire all flashes at the same 
flash output and to ensure that the total flash output results in the 
standard exposure.
Autoflash Photography Using Multiple Slave Units