First, I want to thank you for taking the time to show the plate & L bracket on this type of body. My comments below are my opinion of that arrangement, but I still want you to know that I very much appreciate your feedback you gave!The same way you attach an Arca-Swiss plate to any Canon 1-Series DSLR (or Nikon Dn body). Via the tripod hole on the bottom.
If you want an L-bracket, which also attaches to the tripod hole in the bottom, I'm sure the same folks who make them for the 1D X models will also make them for the R3.
View attachment 196970
OK, but what happens when you do a portrait shot and rotate the camera 90 degrees while keeping your hands in the same position to grip the newly rotated bottom grip? It's now got a huge and awkward plate bolted on it which you're gripping. If you instead rotate your hands to hold onto the original (normal right side) grip in portrait position, then you're using it like an ordinary camera and dumping the use of the 2nd grip. To me this is combination is truly silly, both visually and functionally. I can see people just not using any A.S. plate and saying they handhold it, or else they just screw in a normal (non quick release) monopod or tripod and leave it on for a while. But if people want what you've shown, then good for them, but it I'll pass on that configuration.
This is why I really wish they would have a version of this camera with an integrated bottom grip, and a 2nd version without it.
Now, if they had embedded 2 A.S. grooves into the right side grip and another set into the bottom grip, in such a recessed way as to feel "normal" in your hands then this would have been a brilliant design. You could use it by hand or by hooking up to an A.S. mono- or tripod without adding anything in both landscape & portrait mode without rotating your hands. Then I'd have been quite happy with that design. Also, they could have 2 A.S. grooves recessed into the bottom & left side of a body without the integrated bottom grip so you don't need an add-on plate with removable L bracket with it either (and now you don't have any L plate to interfere with the fully articulating screen).