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.
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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.