I'd imagine the magnesium body acts as a Faraday cage blocking WiFi signals , even with a plastic window as this would only let the signal out in one direction so an external WFT would be more reliable.I do wish we would have had a better look at the port-side of the R3.
My best guess:
View attachment 197932
It's barely legible but I do believe looking at the side that Canon will have chosen to put the Mic and Headphone jacks in the same format as on the R5, to prevent them from being hit by the flip screen when in use. That would also explain why the 3-pin intervalometer was forced onto the front of the camera.
After that, it's all a toss-up. I don't really see space for the WFT, but the very large rear port cover looks like it could fit the WFT, HDMI, and USB-C ports in one space, since we know from the 1DX3 that USB-c saves a lot of room in the port areas over the older USB ports.
In the R5, Canon also chose to isolate the PC sync from headphones/mics so I wouldn't be surprised if that was in the same place. There's another, isolated, large port to the rear of that one that appears to have a pretty square look to it, and I think that would likely be the ethernet port.
Another option, which I really hadn't thought about, is, what if Canon integrates the WFT into the camera? The WFT-R10 was a battery grip with full wi-fi features, perhaps Canon could just include top-of-the-line WIFI connectivity within the battery grip area? I could imagine they could make a "window" of plastic under the rubber grips within the metal shell of the battery grip to give it better range/connection. Total speculation, though, and I doubt Canon would want to lose the sales of the $600 WFT adapters.