Canon REBEL T3I EOS 600D User Manual page 197

Fully-automatic Shooting with the Built-in Flash and
Multiple External Speedlites
The built-in flash can also be added to wireless flash shooting explained
on pages 195-196.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups of multiple
Speedlites complemented with the built-in flash.
[1All and 2]
Basic settings:
1
Flash mode
E-TTL II meter. : Evaluative
Wireless func. : [0+3]
Channel
Select [Firing group].
2
Select the firing group, then set the
flash ratio, flash exposure
compensation, and other necessary
settings before shooting.
A
Custom Wireless Flash Shooting
: E-TTL II
: (Same as slave units)
[1 (A:B) 2]
B
197