Don't think I've ever seen Jean as a mn: Jane, yes, often, but not Jean or Joan. I like it as a fn - it was never popular enough here to be dated, IMO.
I have seen all of them quite a bit. I think June was the least common one (middle name) but that will soon change and is not true for the new generation, I am sure.I've seen Jean, Joan and Jane a lot and even Jayne.Hermione Granger's middle name is Jean, which is nice.