I see a 3rd hole, all right, but I do not know what it does, so I will ask Tiesda about this.
Did you already get your new Rio boards, or did you read about or see the 3 fin holes online
I'm pretty sure the extra hole in the photo may do something else.
In order for it to be called a "Tuttle finbox" (either std. or Deep) it would have to have the
same width and angles, so I'm pretty sure you can use a std.Tuttle fin without any issues
beyond the normal "making it fit right".