I just wrote a post (and deleted it) about my own four adult kids...

You are not alone with the frustration of seeming to have to parent for the rest of your life. I'm tired. Really tired. I have four and it surprises me that even though they were all raised the same, they are all totally different. And I don't just mean in the obvious ways. One is more sensitive, one is more ambitious, one has more internal struggles than another, one I have to watch like a hawk because she won't verbalize her worries, the list goes on and on.

I wanted you to know that you are not alone and I'm sorry that I have no advice for you, except that you are doing the best you can and keep doing it.
They sound like good kids and probably the youngest is taking a little longer to mature than the other 2.