She's specifically one soul inhabiting three fully separate bodies, with it stated that each body can experience things individually which add up to a mental whole, like eating food in each body amplifying the combined feeling.

So each body would be separately impregnated.
All three bodies having triplets in this comic is just for the sake of continuing the "3" theme.

Wait they're all one demon right? Just with three split bodies. So if one gets pregnant do the others? Would the child output really be tripled? What matters here, body count or the fact they're the same demon?