Do these 3 mallow relatives hybridize with each other?
Hibiscus, Hollyhock, Rose of Sharon hybrid?
The hollyhock won't hybridize with the others, it's too distantly related. As for hibiscus and RoS, I haven't heard of this happening and I know it's been tried.
That would be great though, if the RoS was able to be crossed with H. rosa-sinensis - blooms like the hibiscus, with the hardiness of the RoS. I'd buy that!