This isn't a direct answer, but I think it'll help understand their relationship/how similar they're likely to be. All of the above are descendants of an OS th

This isn't a direct answer, but I think it'll help understand their relationship/how similar they're likely to be. All of the above are descendants of an OS that Berkeley released called "BSD". BSD only ran on PDP-11s, and then, VAX minicomputers, with a few proprietary ports to expensive workstations (SunOS, Sony NEWS, etc). The code was a "distribution" of modifications on top of AT&T UNIX, so it couldn't be freely shared to people without a UNIX license. So the developers released part of their own code separately, the most important released called "Net/2". Two alumni of Berkeley worked on expanding the free distribution to be 1) a full OS and 2) run on regular Intel 80386 PC clones. This was called "386BSD". The last "official" BSD was 4.4BSD, which had the goal of wider software and hardware compatibility, adding support for a wide range of computers. Disagreements with the developers of 386BSD led to a fork, eventually named "FreeBSD" (as in Free86BSD, I think). They continued the goal of a PC compatible BSD. Another group took 386BSD and reintegrated 4.4BSD code back in, continuing the Berkeley goal of "wide hardware compatibility", which became NetBSD. "Internal disagreements" eventually led to one of the core developers leaving to make OpenBSD, which differed in that the development source repository was always publicly available (at the time NetBSD only released source for a full release, only team members could pull from the CVS repo). All that to say, FreeBSD is sort of its own thing, with NetBSD and OpenBSD sharing much more. They've all diverged quite a bit, but have simultaneously shared a lot of code (especially things like device drivers). So, broadly speaking, using any BSD will make you "familiar with BSD", but they really are quite distinct at this point.