On Monday, Apple has pulled the signing plug on its iOS 9.3 firmware for its lineup of iPhones, iPod touches, and iPads.
As a result, downgrades through iTunes to iOS 9.3 are no longer possible, and there's no way to upgrade specifically to iOS 9.3 from an earlier firmware.
Instead, users will be directed to install iOS 9.3.1 when restoring or updating their devices through iTunes.
Update: It seems that not all versions of iOS 9.3 have stopped being signed. It appears that iOS 9.3 is not being any longer for newer devices (i.e. iPhone 6s, iPad Air 2), but it is still being signed for older devices (ie. iPhone 5s).