KeePass is the original project. It is an open source dotnet application. The source code, as of the last time I checked, is released as a tar/zip with the binaries only.
KeePassX is an open source c++ application. It was one of the cross platform applications to manage KeePass databases. It has not been under very active development for a while.
KeePassXC is a fork of KeePassX which is under active developement. They have added many features and improvements and has stayed up to date with kdbx updates.
KeePassXC is fantastic, I especially like the good Yubikey integration. The one small feature I miss from the original KeePass was the password templates. I like all my passwords to follow a grouped pattern so its easier to type into a phone or something while still being strong.
Is KeePassXC also audited? The main KeePass has had multiple security audits, also thanks to the EU who funded it one time. Seems relevant for a password manager.
While I like KeePassXC more, this is even more relevant as KeePassXC is made in C++, while Keepass is made in C#. C++ makes it easier to shoot yourself in the foot.
The difference is the UI. They all use the same* database format underneath.
Thr original Keepass was at first a Windows only app, so KeepassX was created as a cross platform reimplementation using Qt. Nowadays Keepass uses .Net and also is cross platform but Linux users prefer Qt because it feels more "native" than .Net.
KeepassXC is a more recent fork of KeepassX, which added a bunch of extra features. Notably, it added support for version 4 of the keepass database format, and it also allows you to use a browser extension to enter passwords inside web browser forms without needing to cut and paste.
I'm a LastPass user trying to switch to KeePassXC. I tried it once on my computer but couldn't find a way to create folder inside a folder inside a folder and so on. I like organizing everything so this is the only thing holding me from switching to KeePass. BTW is it possible to do that in KeePass? I haven't explored it a lot.
I've got a complex hierarchy of folders (Groups in Keepass terminology) in some of my Keepass files. It's definitely possible to do, some of the UIs are better than others at making it obvious.