There were rumors last week that Apple would at last roll out the final version of iOS 11.3 on March 27th, during its special "Let's Take a Field Trip" event in Chicago. This didn't happen, unfortunately, but the very next day Apple did seed the final release of iOS 11.3 for the 9.7" iPad model that was announced at the event and went on sale yesterday.
MacRumors reports, but those of you who'll get their hands on in the coming days will be glad to hear that Apple has already made the long-awaited iOS 11.3 update available for the device. So, when you get yours, simply go to Settings > General > Software update to receive the OTA update.The first orders of the new iPad won't be delivered until later this week,
This is arguably the most important iOS 11 update yet, as it addresses a number of issues, such as the security flaw that allows Siri to read private notifications aloud, and introduces new battery management settings that provide users with more granular information about their batteries. The update also marks the debut of ARKit 1.5 and a number of other features, all of which you can check out here.
As to why Apple seeded iOS 11.3 for the new iPad only and nothing else, it's anyone's guess. The developer preview of 11.3 has been out since January, while the public beta has been available for weeks.