It's a 3/8ths of an octagon type bay, but you're right. 3 small cast iron rads could look ok, but kinda fussy. I would just like it out from behind the sofa. A column might work (in a different spot).
I cheaped out and got three smaller rads to fit with some fiddly pipe work in between, looks pretty tidy for the money. Will update with a pic in the morning.
They make curved ones for bay windows. Might look kinda cack though.