Apple has released yet another update for iOS 9.3 for older devices affected by activation issues that cropped up in the original release last week. This new build, 13E237, corrects the activation issues in older devices, and does not need to be installed on iPhones and iPads that successfully installed a previous build.
Apple originally paused the update to iOS 9.3 for older devices after several owners of such devices had trouble activating them after the original update. The company issued a separate update for the iPad 2 late last week. Note that if you've experienced activation issues, you'll need to install the update through iTunes, and you might need to follow the steps outlined in this Apple support document.