You aren't going to see OpenBSD share a kernel with anyone - it's too different and makes trade-offs the others won't accept. And NetBSD doesn't need the heavyweight kernel FreeBSD uses.
From what I've seen, the BSD community swaps code around on a regular basis. But they pick and choose what code to use based on their own goals. It seems to work pretty well.
From what I've seen, the BSD community swaps code around on a regular basis. But they pick and choose what code to use based on their own goals. It seems to work pretty well.