Id like to add that some of the differences are AFAIU:
Appimage - Distributed. Non-updating - like exe files in Windows or dmg in macOS. By default doesnt have any sand-boxing. Therefore less safe install stuff, but also less of a hassle when youre not used to sandboxing.
Snap - Centralized - So maybe software is checked better before becoming available, but theres also a single chokepoint for stuff getting in. Updating built in. Sandboxed.
Flatpak - Distributed. Updating built in. Sandboxed.
Viewing post in Universal cross-distro itch.io Linux deployment
Snap is centralized, as in Canonical has control over all apps that want to become snap packages? If so, that's not a real standard, and should definitely not be considered for use! You definitely should not need to ask Canonical for permission to package and distribute your own application on your own or any other website.