Perhaps more troubling

1. Perhaps more troubling, one-third of these diagnoses occur in children under age six.
2. Perhaps more troubling is that one-third of these diagnoses occur in children under age six.
2. It is perhaps more troubling that one-third of these diagnoses occur in children under age six.

I would like to know the meaning if "1" is "2", "3", or anything else.
I wonder why "it, is, that" is omitted.