Iris is gorgeous! I also like
Zinnia, but I've never been able to warm up to
Hazel.
When I hear the name
Hazel, I tend to think of the color and hazelnuts before I think of the tree.
Iris and
Zinnia strike me as botanical. If one happened to be a middle name and the other a first name, I think it would be better, but I don't think that it's that big of a deal.
Zinnia isn't the most common flower name, so it's not like the sibset is something really obvious like
Lily,
Rose, and
Daisy. I personally would be a little hesitant to use
Iris and
Zinnia together as first names to avoid making the sibset too theme-y though.
If someone were to use
Iris,
Zinnia, and
Hazel, I'd notice that the parents used two names that I really liked, and that would please me. I guess I'd think that the parents just liked the names and probably the flowers as well. I might wonder if
Iris and
Hazel were named after family members, but older names are coming back, so I think my first guess would be that the parents just liked the names.