Canon Speedlite Transmitter ST-E3-RT Instruction Manual page 19

Wireless Multiple Flash Shooting
You can divide the slave units into two or three groups and perform
E-TTL II/E-TTL autoflash shooting while changing the flash ratio
(factor). In addition, you can set and shoot with a different flash mode
for each firing group, for up to 5 groups.
Autoflash Shooting with Two Slave Groups
Autoflash Shooting with Three Slave Groups
A
C
A
' Wireless Flash Shooting
(p.31)
B
B
(p.32)
17