Snap: An alternative packaging management system produced by Canonical, the company that makes Ubuntu.
Flatpak: An alternative packaging management system produced by the Gnome-Foundation.
APT: Advanced Package Tool, the primary standardized package management used by Debian and all Debian-based systems including Ubuntu.
Any Computing environment is composed of a large number of independent packages that rely on the other components of the system. From the ECU in your car, to a cell phone to a desktop pc running Mac, Windows or Linux.
Installing any new software or package means that the installed software must depend on other existing software.
Even portable stand-alone applications like applets depend on a display manager, windowing manager and kernel.
These dependencies must be met in order to successfully install software.
When you install software on Controlled and Standardized systems like Windows, Mac or Android, you very rarely run into dependency issues (though you can and will) because the standardization reduces the likelihood of a user having variation on their computer.
But on Linux, we rely heavily on FOSS and independent development. While the other systems shun variety, Linux embraces it.
The other systems take your control over your own machine away in order to make using their system easier on you. And... so they can profit, of course.
Linux restores control back to you. With that control comes responsibility for you to manage your system.
What Snap and Flatpak do... is they remove that control. They create confusion in new users migrating from a standardized system. New users often are confused about why there is Snap, Flatpak and APT as different options. This is exacerbated by the control exerted by Canonical and Gnome-Foundation as they demand that Tech Articles promote Snap and Flatpak.
You can see this plainly by running a net search on how to install a piece of software. You will find many search hits leading to articles that tell you to install it as a Snap. Install it as a Flatpak. And at the end of the article, some might condescend to saying... "Oh... and you could use APT too if you wanna..."
There are three versions of FF because one is a Snap, One is a Flatpak and One is APT.
The Snap and Flatpak will be the Latest and Greatest and force-fit by including all possible dependencies with them. APT includes the stable version, fitted to your actual system.