It looks like the iOS 9.3.3 beta profile has expired and Apple has chosen not to renew it. See the Downloads page; there is a profile available for the iOS 10 beta, but not the iOS 9.3.3 beta. It seems that you will have to install the iOS 9.3.3 beta 4 via iTunes instead of OTA now.
EDIT: Forgot to add, when I installed beta 4 from iTunes using Option - Check for Updates (as described in the "iOS beta Software Installation Guide" at the bottom of the downloads page), iTunes gave an error message about an "invalid reponse" when the device restarted after the install. However, the installation was fine, and I did not have to restore from a backup - all my apps and data were still on the device.