No harder than getting it right in native code on 3 different platforms.
PhoneGap development is hard inasmuch as there are tons of gotchas you don't know about and haven't been clearly and extensively documented.
I agree that supporting multiple platforms is suboptimal, but instead choosing a suboptimal common platform isn't any better.
We already tried that once with Java desktop applications, and Java at least tried to provide a viable common widget/platform toolkit.