They are all viable as backend operating systems, understood as server use. I don't think it's uncontroversial to say that it what they are mainly used for, although some people also use them on desktops/laptops.

I'm not terribly familiar with NetBSD and FreeBSD anymore, but I can provide some of my own motivation for using OpenBSD on servers whenever I am able. I don’t consider myself a beginner to Unix or computers, but I do consider myself a fairly inept and inexperienced systems administrator, with no great desire to spend the time to become better, and my needs are fairly basic - just the usual web/shell/IRC/mail server stuff, and other random infrastructure needs that come along for my work. I prefer OpenBSD because it is is so minimalist, the defaults so sensible, and the documentation so good, that I trust the machines I set up. I have confidence that I did not overlook something crucial. The OpenBSD http daemon is beautifully simple - too simple for many uses, but perfect for mine. The OpenBSD mail daemon is the only mail daemon I have ever been able to set up from scratch, just from reading the man pages. Other systems are more featureful (and probalby faster), but I always worry that I might have overlooked something.

I run Linux (NixOS and RHEL) on my desktop and some servers respectively, because of needs that OpenBSD simply does not support (mostly GPU computing). Linux is fine and certainly runs very fast, but OpenBSD is the only operating system I honestly like.

I've heard someone mention that FreeBSD is "the Linux of the BSDs", meaning that it is bigger, has more features, and is faster, but (probably) also that it is more complicated. I don't know if that is actually true. My last FreeBSD experience was in 2003, where I used it because the Linux emulation could run 3D games like Enemy Territory faster than Linux itself could. (I don't think that is a common use case anymore.)