All three serve the same purpose, but they are drastically different. For starters, WP User Manager and BuddyPress are plugins, while BuddyBoss is a theme and plugin.
The second difference is in capability. BuddyPress gives you the tools to turn your website into a community quickly and with ease. BuddyBoss gives you a colossal interface to toy around with and create as intricate of a social network system as you like, and WP User Manager gives you both of these.
The third and most prominent difference, which is the point of this article, is the ease of use. BuddyPress and WP User Manager are both relatively easy to use, which we can't say about BuddyBoss. Don't get us wrong, BuddyBoss is still a relatively simple theme and plugin – it has so many features that it can be harder to use