Apple on Tuesday issued third betas of iOS 10.3.3, macOS 10.12.6, watchOS 3.2.3, and tvOS 10.2.2 to the developer community. iOS 10.3.3beta3 is also available in latest 3uTools V2.12.

Step1: Download and install the latest 3uTools on your PC. Before upgrading, you'd better backup your iDevice. Connect your iPhone to PC with an USB cable. Then head to "Flash & Jailbreak” section in 3uTools, click "Easy flash". 

Note: If you want to retain the data, please select the option "Retain User's Data While Flashing".

Step2: Select the latest iOS 10.3.3beta3 on this page, and then click "Flash" button. 

Step3: Keep a good connection between iDevice and PC, and then just wait until this process is finished.