Looking at the shoulder, the two-button layout actually makes sense.
On the 1DX, the front button is "mode," second button is "drive-AF," third button is "flash exp comp and metering mode" with pressing both DRIVE-AF and Mode at the same time giving you exposure bracketing.
On the R3, the mode button is in the top control wheel, so they've simply removed the mode button from here and changed it so that you press both DRIVE-AF and the Flash Comp/Metering mode buttons together to get to exposure bracketing.
I'm glad to see that hasn't changed on the R3!
As for the right shoulder,
We see that this is a very similar layout to the EOS R5, however, the record button is on the rear plate now, and the lock button is in the mode-switch on the rear plate, like on the 1DX.
So we have two buttons by the top control wheel, what is clearly the LCD illuminator button on the right, and then a new button on the left, where the record button used to be.
It looks to me as a solid-ish block icon, so I'm leaning towards this button being exposure compensation? On my 1DX I don't personally use any of the top buttons often, but ISO can now be controlled by the top wheel or the lens control ring, so I doubt it's ISO. The other option would be WB, but I suspect that if they had to get rid of a button, not way too many people change WB constantly, and there's many ways to change that now in the camera's Q button and viewfinder, so I don't expect it to be WB. Exposure compensation can be done by the control wheel, sure, but if you set it to ISO control you can't. Exposure compensation is also a button I suspect some pros need to hit many times a day, so I'd expect they'd leave it on for the pros who need it for muscle memory while shooting.
At the very least, whatever it is, I kinda suspect it will end up a customizable button, which would be nice for those who don't use exposure compensation much.