You can disable the beta updates in System Preferences / App Store, but if you are already on 10.11.3 I would wait until the final version is released, because you will always stay on Beta 1 if you change it now and don't get any updates with bug fixes since then.
If the option is not listed there you can use a Terminal command to get rid of beta updates:
softwareupdate --clear-catalog
Also you can login to the Apple beta website and completely cancel the beta program:
that beta was just released to the public, so yes, it's the newest version of the OS. if you don't want any betas, opt out. otherwise, you're as up-to-date as possible...