ST-E3-RT with mixed radio slaves

I have been using a Canon ST-E3-RT to trigger my 600EX-RT for a few years now and recently decided to add another speedlight but this time I went for a YN 600EX-RT-II. After I added the Channel and ID that I use to the new flash I tested both out and for some reason the YN is triggering much sooner than the Canon. Aren't they supposed to fire at the same time? Both are radio slaves and I am using the ST-E3-RT trigger in E-TTL mode.

Any advice as to what I am doing wrong?

They should fire at the same time.
Let’s start at the beginning. What type of batteries in all three units, Canon 600, YN 600 and the Canon ST-E3 RT?
AA, Ni-MH etc…

All three have identical alkaline batteries. If I change the mode on the ST-E3-RT from E-TTL to Manual, both flashes fire at the same time. Adjusting the manual power setting for Group A results in corresponding flash outputs as expected.

I haven’t used Alkaline batteries in my “600 system” since day one.
Alkaline batteries. Invest in Ni-MH (Nickel-Metal Hydride)
Low Discharge Ni- MH are better as they don’t loose the charge as fast.
Eneloop or PowerEx are in my arsenal.
Try switching the four batteries in the two Speedlites.

Is your YN flash set properly to wireless radio slave and not optical slave S1?

agv8or wrote in post #19329323
Is your YN flash set properly to wireless radio slave and not optical slave S1?

optical slave would possibly introduce delay because it is triggered by the first flash to fire, but OP is seeing the YN fire 'sooner' than the non-YN which are both triggered by the Canon radio. "YN is triggering much sooner than the Canon"

Oddly, aftermarket flashes usually exhibit greater latency, forcing users to slow down shutter from X-sync speed, while the Canon flash can be fired at the usual X-sync shutter speed. In this case, there is apparently less latency from the YN, than the Canon flash.

Op said he was in Ettl. If yn flash set to optical slave s1 rather than wireless radio slave then yn flash will fire when it sees the preflash from other canon slave. So it’s not delayed rather it will fire before the flash exposure.

agv8or wrote in post #19329592
Op said he was in Ettl. If yn flash set to optical slave s1 rather than wireless radio slave then yn flash will fire when it sees the preflash from other canon slave. So it’s not delayed rather it will fire before the flash exposure.

Point made.

It would be nice if OP came back and confirmed whether or not the Yungnuo flashes were in optical slave mode (rather than radio control).

I had something like this happen to me recently. With my 580EX ii and my 600EX ii RT set to optical master and optical slave, the slave fires late when i press the Pilot/test button. But both fire perfectly when triggered by the camera when taking a photo.

I test to make sure the slave is properly triggered by turning off the master output as well.

chuckmiller wrote in post #19445878
I had something like this happen to me recently. With my 580EX ii and my 600EX ii RT set to optical master and optical slave, the slave fires late when i press the Pilot/test button. But both fire perfectly when triggered by the camera when taking a photo.

That's by design; it makes it easier for you to confirm the function when they flash sequentially.