That hole circled in red, what is it for?
I'm not positive, but I believe the other two holes are for the mounting screws. That would mean that this third, non-beveled hole may be the access port for the grip adjustment screw. It would be much easier to tell if you had another picture of the lever partially assembled.
If it is an access for the grip adjustment, you should be able to peel the rubber cover back and then use that adjustment screw to change the "reach" between the lever and the bar to allow for ease of use and comfort for people with different size hands.
It's also possible that the grip adjustment is in another location and that the hole exists simply to reduce the weight of the part.
Another possibility is that it is some sort of artifact from the injection molding process. I’m not too familiar with what that may entail, but I presume it could be the result of an alignment pin, or perhaps something to ease removal/extraction. I doubt it’s a weight saving thing because there could be a lot more material removed here if weight reduction was the goal; why is there only that one hole then?