It's so you can roll back to the previous version if you want without messing with uninstalling and reinstalling Opera. Chromium and Chrome used to do the same thing but don't anymore.
There should only be 2 folders: the latest and the previous one. However, for unknown reasons, sometimes after an update, the 3rd one doesn't get deleted. And, if that happens again, they can start to pile up. Updating via the installer instead of autoupdate might cause one to remain too.
You can just delete all the older ones if you want so you just have the latest.
Since most users don't like this and don't really have a need for it, hopefully Opera will change to not keep any old versions at all.