Users of older iOS devices were experiencing issues updating to iOS 9.3, but Apple have fixed this, as MacStories reports:

As reported by MacRumors, Apple released an update to iOS 9.3 (build 13E237) a short time ago that addresses the activation problem on older devices. With the new build of iOS 9.3, people with older devices who didn’t previously update to version 9.3 should be able to do so now via an over-the-air update.

I used to think that only the paranoid wait to upgrade, but now I’m either paranoid or realize the wisdom in waiting.