i actually don’t know any of the three well, feature-wise. i have only tried discord and element in the past, but i don’t follow any of these chat based communities these days

both being open source, Revolt AFAIK has its own platform/protocol, so Revolt clients will talk with other Revolt clients

Matrix is not a client, or ‘app’, it’s a protocol, like ActivityPub (that Lemmy uses). these are used by different apps to communicate. for ex. to use Matrix you can either use Element, Cinny, Syphon, etc. and they will all talk with each other, even if each one of them has its own features/UI