Here's my short version from rather limited knowledge.

FreeBSD is oldest and largest of the three, started in the ballpark of the late-eighties from ta-da: BSD, which was a Berkeley funded project. Its extremely secure by default. My FreeBSD machine only has 4 ports open offhand: smtp (sendmail), ssh, X, and er... I think its either time or printer.

NetBSD was a fragment that occured I think originally over architecture support. Regardless, the goal of NetBSD seems to be protability to as many hardware architecture platforms as possible: ppc, x86, alpha, sparc, VAX, pdp11's, tomagochis, dreamcast, the ps2, your toaster... its something like 120+.

OpenBSD fragmented off of NetBSD I think because its founder, Theo de Raadt, basically had issues with everything and needed to start his own OS. (read: meglomaniac) Whatever, OpenBSD brags (truthfully) that there has not been a remote root hole found in the default install in 4 years (which is 8 versions, they keep a solid timetable between releases). To give you an idea, openssh, was written for OpenBSD.

To check out any of them, just tack .org onto the end of thier name. OpenBSD is a headache to get isos for as that's the only thing that's covered under a closed copyright.

-Cheers,

Finegan