freebsd = utility: performance, gnu software compatibility, drivers, closest thing to useful as a desktop (still not very), but almost x86 only

openbsd = security: internet exposed services

netbsd = portability: already ported / most easily portable to every cpu arch & platform including old and new obscure ones. If you want a big-endian os for whatever academic reason, it's netbsd on an old sun sparc or pi3. I had an HP Jornada that had a SH3 cpu and shipped with wince, the only alternative os on that was netbsd.