Where would people find this out? Has anyone listed the BSDs and said which task each was meant for? Are there any good BSD-to-BSD comparisons being done now?
Lots of people have listed the BSDs and said which task each was meant for. Lots of people have also been completely wrong. There's a whole internet of misinformation out there if you're really interested.
Yes, of course, but there is a general aim and focus among the developers of the three main flavors, that is prevalent in the capacities of said flavors:
With that said, no, that definitely does not mean that each flavor can't stand out in the other two fields. Heck, I used OpenBSD as my desktop OS for 2 years on and off, so I know very well how capable it is "off field", and looking at the most popular flavor in BSD-powered server parks we see FreeBSD instead of OpenBSD contrary to what logic might try to dictate.