The EOS 70D only has ratio control for 2 groups, A&B, however Canon Speedlites can be triggered if those flashes are set to individual slave mode.
Using the optical wireless system a slave can be set to manual mode by pressing and holding down the mode button on the flash for a few seconds. This will put the slave in manual and the group letter will not be displayed. If a Speedlite 600EX-RT or 600EX is used in optical wireless mode the display will show individual slave.
As an individual slave you set the manual power level on the flash. The flash will fire in sync with the other flashes being triggered even if they are using E-TTL automatic flash.
In short a slave set to individual slave mode will react to only one command from a master flash - the "fire now" command. All slaves in any mode, E-TTL or manual react to this command.