Here is the concept art for the 3 kaiju revealed so far.
According to the official site, they are called, in order, Ghoulghilas, Dévadan and Anti. If you look at the designer, it's listed as "? ? ?". This is because they weren't designed for the anime, but are taken from the original Gridman.
Ghoulghilas: Now, for these guys I couldn't find a clear direct 1-to-1 equivalent. They seem closer to this guy from Ultraman, but this one from Gridamn has that same body structure too. I'll ley you guys judge.
Dévadan ("Giorgio" in the original): The clearest one for me.
Anti ("Shinobilar" in the original): 1 2 3
Just something I found yesterday and wanted to share, cheers!