Apple has released iOS 11.3. However, it is only for the new 9.7-inch iPad announced this week. Once you get your hands on one, you can download iOS 11.3 over-the-air using the Settings app. To access the update, go to Settings -> General -> Software Update.
Hopefully iOS 11.3 will roll out ASAP for other iPads and iPhones. IOS 11.3 introduces new features, such as iCloud notifications for iMessages storage in iCloud, ARKit 1.5, and a "Battery Health" feature designed to give iOS users more information about the batteries.